Add support for the PowerPC 64-bit SVR4 ABI.

The Link Register is volatile when using the 32-bit SVR4 ABI.
Make it possible to use the 64-bit SVR4 ABI.
Add non-volatile registers for the 64-bit SVR4 ABI.
Make sure r2 is a reserved register when using the 64-bit SVR4 ABI.
Update PPCFrameInfo for the 64-bit SVR4 ABI.
Add FIXME for 64-bit Darwin PPC.
Insert NOP instruction after direct function calls.
Emit official procedure descriptors.
Create TOC entries for GlobalAddress references.
Spill 64-bit non-volatile registers to the correct slots.
Only custom lower VAARG when using the 32-bit SVR4 ABI.
Use simple VASTART lowering for the 64-bit SVR4 ABI.

llvm-svn: 79091