[PowerPC][LLD] Make sure that the correct Thunks are used.

This fixes an issue where mixed TOC / NOTOC calls can call the incorrect
thunks if a previous thunk already exists. The issue appears when a TOC
funciton calls a NOTOC callee and then a different NOTOC function calls the same
NOTOC callee. In this case the linker would sometimes incorrectly call the
same thunk for both cases.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D101837