[ELF][PPC32] Make R_PPC32_PLTREL retain .got

PLT usage needs the first 12 bytes of the .got section. We need to keep .got and
DT_GOT_PPC even if .got/_GLOBAL_OFFSET_TABLE_ are not referenced (large PIC code
may only reference .got2), which is the case in OpenBSD's ld.so, leading
to a misleading error, "unsupported insecure BSS PLT object".

Fix this by adding R_PPC32_PLTREL to the list of hasGotOffRel.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D114982