Makefile.in: pass ARCH for modules_install as well

To do a cross-build using only kbuild rather than a full source tree,
ARCH= needs to be passed for the kbuild Makefile to find the
archspecific Makefile.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #16944