[COFF] Align import address chunks to the pointer size

This fixes cases on ARM64 when importing from more than one DLL,
in case the imports from the first DLL ended up unaligned.

When fixing up a IMAGE_REL_ARM64_PAGEOFFSET_12L, which shifts the
offset by the load/store size, check that the shift doesn't discard
any bits. (This would also detect if the import address chunks were
unaligned.)

Differential revision: https://reviews.llvm.org/D35640

llvm-svn: 308585