This patch utilizes getReservedRegs() to find asm clobberable registers.
And to make the result of getReservedRegs() accurate, this patch
implements the todo, which is to make r2 allocatable on AIX for some
leaf functions.
Utilize common API in PPCTargetParser
(https://github.com/llvm/llvm-project/pull/97541) to set default CPU
with same interfaces for LLC.
This will update AIX default CPU to pwr7 and LoP powerppc64 default CPU
to ppc64.