gas: Restore tc_pe_dwarf2_emit_offset for pe-aarch64

Restores tc_pe_dwarf2_emit_offset in tc-aarch64.c, which is needed to
make sure that DWARF offsets are encoded correctly (they're secrels in
COFF). There were remnants of this there before, but they were removed
by Jedidiah's original patch - presumably because we didn't yet have
.secrel32.