[ELF][AArch64] Add missing PLT relocations to isStaticLinkTimeConstant

r347650 fixed pr38074 for AArch64 for static linking. It added two new
RelExpr instances R_AARCH64_GOT_PAGE_PC_PLT and R_GOT_PLT. These need to be
added to isStaticLinkTimeConstant so that the address of an ifunc can be
taken when building a shared library.

fixes pr40250

Differential Revision: https://reviews.llvm.org/D56666

llvm-svn: 351186