Both Debian and kernel archs are "arm64" but UTS_MACHINE and gcc say
"aarch64".  Recognizing just the latter should be enough but let's
accept both in case something regresses again or an user sets
UTS_MACHINE=arm64.

Regressed in cfa88c79: arm64: Set UTS_MACHINE in the Makefile.
Signed-off-by: Adam Borowski <kilobyte@angband.pl>
Acked-by: Riku Voipio <riku.voipio@linaro.org>
Signed-off-by: Michal Marek <mmarek@suse.com>