Support PIC relocation model

[jbush001]

Currently backend is hardcoded to use static linking model. Add support to compile as position independent code. This would save allow loading shared libraries in an efficient way.

• NyuziISelLowering::LowerGlobalAddress should call TargetMachine::getRelocationModel and generate code to access global offset table (GOT).
• NyuziISelLowering::LowerCall should check relocation model and use procedure linkage table (PLT) if necessary
• Create GOT and PLT fixup types and relocations. Add code to NyuziElfObjectWriter::getRelocType to handle.
• In NyuziMCInstLower::LowerSymbolOperand, may need custom code to generate GOT and PLT symbol types
• In NyuziMCCodeEmitter, add code to generate fixups
• In tools/lld, add relocation handling for GOT and PLT types.
• Switch lookup tables need to be made offsets instead of absolute addresses.

[jbush001]

There isn't a use case for this, since it requires shared libraries and an OS, which don't exist yet.

[jbush001]

Status: Working:

• Global addresses are emitted with GOT fixups, the linker can will create the GOT section and relocate indices.

Not working/to do:

• Relative offsets for switch statements (needs addend)
• Function prologue code to initialize GP
• External call support/PLT

[jbush001]

Switch statements fixed in 37f47d83889d00ae251c3a5ae9450db128874b3d.

[jbush001]

Call addresses are now in the GOT: 26b78085e557028f8e4d239d84d6ab3afff1313a