kvmtool: Makefile: Don't defer error output on unsupported architectures

Having ARCH defined to something unknown or trying to compile kvmtool
on an unsupported architecture barfs quite late, actually after
checking for libraries. This gives a lot of messages, but the only
useful one (architecture not supported) is easily overseen.
Abort early if ARCH contains an unknown architecture name.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>