selftests: vm: enable cross-compilation

Selftests vm builds break when doing cross-compilation.  The Makefile
MACHINE variable incorrectly picks upp the host machine architecture.

If the CROSS_COMPILE variable is set, dig out the target host architecture
from CROSS_COMPILE, instead of calling uname.

Link: https://lkml.kernel.org/r/20230109114251.3349638-1-bjorn@kernel.org


Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>