Add support for Linux on aarch64

[lopsided98]

This PR adds support for Linux on aarch64, which requires a number of changes besides the arch specific header.

Being a more recently architecture, Linux on aarch64 does not implement older syscalls such as open, fork and dup2. I added compatibility wrappers that emulate fork using clone and dup2 using dup3. I replaced open with openat, which I believe is available on all supported systems.

aarch64 also does not support certain ptrace requests, such as PTRACE_GETREGS, PTRACE_SETREGS, PTRACE_PEEKUSER and PTRACE_POKEUSER. Instead, PTRACE_GETREGSET and PTRACE_SETREGSET must be used for all register manipulations. I believe these are available on all supported systems, so we shouldn't need special cases for aarch64.

I also switched from using the user struct to using user_regs_struct/user_regs/pt_regs depending on the architecture. aarch64 does not have a user struct, and I felt that it would be more clear and consistent to use just the register struct on all arches. AFAIK, ptrace only fills the register part of the user struct anyway.

I have tested these changes under Linux on x86_64, armv6l, armv7l, aarch64, and powerpc.

I attempted to test on FreeBSD 11.2, but running reptyr (even without this PR) results in a kernel panic. Besides whatever is wrong with reptyr, an unprivileged user being able to cause a panic seems like a fairly serious FreeBSD bug.

Panic backtrace

``` Fatal trap 12: page fault while in kernel mode cpuid = 0; apic id = 00 fault virtual address = 0x2b0 fault code = supervisor write data, page not present instruction pointer = 0x20:0xffffffff80b60eea stack pointer = 0x28:0xfffffe007af43760 frame pointer = 0x28:0xfffffe007af43880 code segment = base 0x0, limit 0xfffff, type 0x1b = DPL 0, pres 1, long 1, def32 0, gran 1 processor eflags = interrupt enabled, resume, IOPL = 0 current process = 747 (reptyr) trap number = 12 panic: page fault cpuid = 0 KDB: stack backtrace: #0 0xffffffff80b3d567 at kdb_backtrace+0x67 #1 0xffffffff80af6b07 at vpanic+0x177 #2 0xffffffff80af6983 at panic+0x43 #3 0xffffffff80f77faf at trap_fatal+0x35f #4 0xffffffff80f78009 at trap_pfault+0x49 #5 0xffffffff80f777d7 at trap+0x2c7 #6 0xffffffff80f5769c at calltrap+0x8 #7 0xffffffff80b5fa3d at sys_ptrace+0x18d #8 0xffffffff80f79038 at amd64_syscall+0xa38 #9 0xffffffff80f57eed at fast_syscall_common+0x101 ```

[lopsided98]

Are you planning to release a new version sometime? Having a tagged release would be helpful in getting new changes to be accepted downstream by Linux distributions.

[nelhage]

Yeah, I'm happy to tag a new release. I'll do so sometime this week.

[nelhage]

@lopsided98 I tagged 0.7.0!

[lopsided98]

Thanks!