[lld-macho] Diagnose unaligned arm64 PAGEOFF12 relocations

The LDR and STR instructions store their immediate offsets as a multiple
of the load/store's size. Therefore, if the target address is not
aligned, the relocation is not representable. We now emit an error if
that happens, similarly to ld64.

This commit removes a test case from loh-adrp-ldr.s that contained an
unaligned LDR.

Differential Revision: https://reviews.llvm.org/D133269