[ELF][PPC64] Remove unneeded PPC64PCRelLongBranchThunk

This reverts the PPC64PCRelLongBranchThunk part from D86706.
PPC64PCRelLongBranchThunk is the same as PPC64R12SetupStub.

Use `__gep_setup_` instead of `__long_branch_pcrel_` for the stub symbol name
as it more closely indicates the operation.
(Note: GNU ld uses `*.long_branch.*` and `*.plt_branch.*`).

Reviewed By: NeHuang, nemanjai

Differential Revision: https://reviews.llvm.org/D114656