[llvm][ELF][AArch64] Handle R_AARCH64_PLT32 relocation

This patch allows for usage of the @PLT modifier in AArch64 assembly which
lowers to an R_AARCH64_PLT32 relocation. See D81184 for handling this
relocation in lld.

Differential Revision: https://reviews.llvm.org/D81446