[build] Conditionalise use of -mabi=lp64 for ARM64 builds
The -mabi option was added in GCC 4.9. Test for the existence of this
option to allow for building with earlier versions of GCC.
Reported-by: Benjamin S. Allen <bsallen@alcf.anl.gov>
Signed-off-by: Michael Brown <mcb30@ipxe.org>