[COFF, ARM64] Fix symbol offsets in ADRP/ADD/LDR/STR relocations

In COFF, a symbol offset can't be stored in the relocation (as is
done in ELF or MachO), but is stored as the immediate in the
instruction itself. The immediate in the ADRP thus is the symbol
offset in bytes, not in pages. For the PAGEOFFSET_12A/L relocations,
ignore any offset outside of the lowest 12 bits; they won't have any
effect on the ADD/LDR/STR instruction itself but only on the associated
ADRP.

This is similar to how the same issue is handled for MOVW/MOVT
instructions in ELF (see e.g. SVN r307713, and r307728 in lld).

This fixes "fixup out of range" errors while building larger object
files, where temporary symbols end up as a plain section symbol and
an offset, and fixes any cases where the symbol offset mean that
the actual target ended up on a different page than the symbol
itself.

Differential Revision: https://reviews.llvm.org/D35791

llvm-svn: 309105