[ELF][PPC64] Enable R_PPC64_REL14 trunks

The thunk implementation is available but an assertion disallows it.
Linux kernel has such a use case: in arch/powerpc/kernel/exceptions-64s.S:handle_page_fault,
beq+ ret_from_except_lite may get out of range.

Link: https://github.com/ClangBuiltLinux/linux/issues/951

Differential Revision: https://reviews.llvm.org/D76904