[NFC][PowerPC] Cleanup 64-bit and Darwin CalleeSavedRegs

Summary:
- Remove the no longer used Darwin CalleeSavedRegs
- Combine the SVR464 callee saved regs and AIX64 since the two are (and should be) identical into PPC64
- Update tests for 64-bit CSR change

Reviewers: sfertile, ZarkoCA, cebowleratibm, jasonliu, #powerpc

Reviewed By: sfertile

Subscribers: wuzish, nemanjai, hiraditya, kbarton, shchenz, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77235