Commit 1a7746603 ("[build] Fix use of inline assembly on GCC 4.8 ARM64
builds") switched from using "%c0" to "%a0" in order to avoid an
"invalid operand prefix" error on the ARM64 version of GCC 4.8.

It appears that the ARM64 version of GCC 8 now produces an "invalid
address mode" error for the "%a0" form, but is happy with the original
"%c0" form.

Switch back to using the "%c0" form, on the assumption that the
requirement for "%a0" was a temporary aberration.

Originally-fixed-by: John L. Jolly <jjolly@suse.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>