Support for cross-compiling kernel modules

In order to correctly cross-compile, one has to pass ARCH and
CROSS_COMPILE make flags to kernel module build calls. Facilitate this
in the same way as for custom CC flag by recognizing KERNEL_-prefixed
configure environment variables of same name.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Phil Sutter <phil@nwl.cc>
Closes #16924