[PowerPC]: Don't allow r0 as a target for LD_GOT_TPREL_L/32

Summary:
The linker is free to relax this (relocation R_PPC_GOT_TPREL16) against
R_PPC_TLS, if it sees fit (initial exec to local exec).  If r0 is used,
this can generate execution-invalid code (converts to 'addi %rX, %r0,
FOO, which translates in PPC-lingo to li %rX, FOO).  Forbid this
instead.

This fixes static binaries using locales on FreeBSD/powerpc
(tested on FreeBSD/powerpcspe).

Reviewed By: nemanjai
Differential Revision: https://reviews.llvm.org/D76662