[PowerPC] Don't use the S30 and S31 regs for the pic code

These changes to address issue
https://github.com/llvm/llvm-project/issues/55857.

Since R30/S30 is used as pointer (32 bits) for GOT Table in the ppc32 ABI,
remove it from the SPE callee save register when PIC is enabled.

This prevents emitting the SPE load and store for S30 and S31 regs.

Differential revision: https://reviews.llvm.org/D127495