Makefile: relax arm test

Currently Makefile accepts only armv7l.* When building kvmtool under 32bit
personality on Aarch64 machines, uname -m reports "armv8l", so build fails.
We expect doing 32bit arm builds in Aarch64 to become standard the same way
people do i386 builds on x86_64 machines.

Make the sed test a little more greedy so armv8l becomes acceptable.

Acked-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>