[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