[PowerPC][LLD] Change PPC64R2SaveStub to only use non-PC-relative code

Currently the PPC64R2SaveStub thunk will produce Power 10 code by default.
This produced an issue when linking older code that made use of the st_other=1
bit but was never meant to be linked or run on Power 10.

This patch makes it so that only the R_PPC64_REL24_NOTOC relocation can produce
Power 10 code.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D129580