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

rpm/redhat/zfs-kmod.spec.in

@@ -69,7 +69,9 @@ fi
         %{debuginfo} \
         %{?kernel_cc} \
         %{?kernel_ld} \
-        %{?kernel_llvm}
+        %{?kernel_llvm} \
+        %{?kernel_cross_compile} \
+        %{?kernel_arch}
 make %{?_smp_mflags}
 
 # Module signing (modsign)