Overhaul of CSR functionality

This makes changes to the CSR implementation to make it more modular and more easily expanded for F***a:

It separates out CSR functionality from RevRegFile, moving it to a RevCSR base class of RevRegFile.
Instead of RevCSR being a scoped enum class with names of CSR registers, it is now a full class with an unscoped enum listing the names of all CSR registers. The syntax to access a CSR register is still RevCSR::name.
Instruction decoding macros have been replaced with constexpr functions for better safety. The return type of a decoding is the smallest unsigned integer type capable of representing all values of the decoded field.
In ZeroExt() if the type was smaller than int, it would get promoted to int and that could cause undefined behavior because left-shifting a negative signed value is undefined. I changed the syntax slightly so that it only left-shifts unsigned values.
The RevCore::output member was made public so that error messages could be outputted just knowing the owning RevCore.
Floating-point CSR type declarations were moved out to RevFCSR.h.
A bug in GetFRM() was fixed -- it only returned the bottom 2 bits of the dynamic rounding mode, instead of the 3 bits it contains.
Some header file dependencies were fixed -- previously some files depended on one header file they #included to #include another header file they depended on. Now every header file that a file needs is explicitly #included by it instead of depending on another header file to #include it.
Zicntr (rdcycle, rdtime, rdinstret, etc.) was added as another extension (Zicntr implies Zicsr).
Zicntr was moved into its own header file RevZicntr.h
RevCSR was structured in a way so that handling additional CSR registers is made easy with base classes of RevCSR. RevZicntr is the first example.
To access functions defined in derived classes, such as accessing GetPC() in RevRegFile from RevZicntr, the functions are declared pure virtual (= 0) in the base classes, not as an abstract interface declaration, but to indicate that some derived class must implement it for the base class. Pure virtual function calls do not use dynamic dispatch, so there is less of a performance hit than with regular virtual function calls.
RevRegFile::IsRV64 and RevRegFile::HasD were changed from const data members to member functions, so that they could be accessed from base classes which declare them pure virtual. Since they are just RevFeature queries, R->IsRV64() was changed to F->IsRV64() wherever possible, even though making them pure virtual in base classes should not require dynamic dispatch.
Similarly, RevRegFile::GetCore() was added, so that RevCSR and its base classes could access the RevCore owning the hart by declaring GetCore() as a pure virtual function.
For the floating-point environment handled in RevFenv.h, instead of storing a reference to the FCSR floating-point status and modifying it at RevFenv destruction time, only a pointer to the RevRegFile is stored, and a new function RevCSR::SetFFlags() is used to set floating-point exception flags.
If Zicntr is not enabled, then accessing the TIME, CYCLE, and INSTRET CSR registers will print a fatal error message about Zicntr not being enabled.
If RV64 is in effect, the RDCYCLEH, RDTIMEH and RDINSTRETH instructions will produce a fatal error message about them being illegal instructions on RV64.
SetCSR() was changed to return bool, currently always true. If false, it will make CSR instructions which modify CSR registers produce a fatal non-specific error message.
A zicntr label was added to the tests, so that RDCYCLE, RDTIME and RDINSTRET tests pass the Zicntr architecture string to Rev. Currently Zicntr is not supported by my toolchain, but it is enabled by default with Zicsr. If we move up in toolchains to where Zicntr is not enabled by default, the test Makefiles have commented out -march= flags which will enable Zicntr.

A detailed comment about the design and modularity of Rev's CSR code was added, to prevent confusion:

////////////////////////////////////////////////////////////////////////////////
//
// A note about the separation of scopes in include/insns/Zicsr.h and
// include/RevCSR.h:
//
// RevCSR.h: GetCSR() and SetCSR() are used to get and set specific CSR
// registers in the register file, regardless of how we arrive here. If a
// particular CSR register is disabled because of CPU extensions present, or if
// a particular CSR register does not apply to it (such as RDTIMEH on RV64),
// then raise an invalid instruction or other exception here.
//
// Zicsr.h: Decode and execute one of only 6 CSR instructions (csrrw, csrrs,
// csrrc, csrrwi, csrrsi, csrrci). Do not enable or disable certain CSR
// registers, or implement the semantics of particular CSR registers here.
// All CSR instructions with a valid encoding are valid as far as Zicsr.h is
// concerned. The particular CSR register accessed in a CSR instruction is
// secondary to the scope of Zicsr.h. Certain pseudoinstructions like RDTIME or
// FRFLAGS are listed separately in Zicsr.h only for user-friendly disassembly,
// not for enabling, disabling or implementing them.
//
// To ease maintainability and prevent large code size, it is recommended that
// functions related to specific CSR registers be made base classes of RevCSR
// in separate header files (e.g. RevZicntr). RevCSR can then dispatch the
// GetCSR()/SetCSR() functions of these CSR registers to the base class.
//
// To access a RevCSR or RevRegFile member function in one of its base classes,
// it is recommended that the function be made a pure virtual function in the
// base class so that RevCSR or RevRegFile must override it, similar to how
// GetCore() is a pure virtual function in RevZicntr which RevRegFile
// overrides. RevZicntr needs GetCore(), but rather than have to store a
// pointer in RevZicntr's constructor, it is much simpler to simply declare
// GetCore() as a pure virtual function in RevZicntr which must be overriden,
// which makes RevZicntr and RevCSR abstract classes which cannot be
// instantiated except as a base class of RevRegFile, which defines GetCore().
////////////////////////////////////////////////////////////////////////////////

Cc: @tdysart

tactcomplabs / rev

Overhaul of CSR functionality #327