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.

Signed-off-by: Phil Sutter <[email protected]>

Author: Phil Sutter

config/kernel.m4

@@ -681,11 +681,16 @@ AC_DEFUN([ZFS_LINUX_COMPILE], [
 		building kernel modules])
 	AC_ARG_VAR([KERNEL_LLVM], [Binary option to
 		build kernel modules with LLVM/CLANG toolchain])
+	AC_ARG_VAR([KERNEL_CROSS_COMPILE], [Cross compile prefix
+		for kernel module builds])
+	AC_ARG_VAR([KERNEL_ARCH], [Architecture to build kernel modules for])
 	AC_TRY_COMMAND([
 	    KBUILD_MODPOST_NOFINAL="$5" KBUILD_MODPOST_WARN="$6"
 	    make modules -k -j$TEST_JOBS ${KERNEL_CC:+CC=$KERNEL_CC}
 	    ${KERNEL_LD:+LD=$KERNEL_LD} ${KERNEL_LLVM:+LLVM=$KERNEL_LLVM}
 	    CONFIG_MODULES=y CFLAGS_MODULE=-DCONFIG_MODULES
+	    ${KERNEL_CROSS_COMPILE:+CROSS_COMPILE=$KERNEL_CROSS_COMPILE}
+	    ${KERNEL_ARCH:+ARCH=$KERNEL_ARCH}
 	    -C $LINUX_OBJ $ARCH_UM M=$PWD/$1 >$1/build.log 2>&1])
 	AS_IF([AC_TRY_COMMAND([$2])], [$3], [$4])
 ])

module/Makefile.in

@@ -55,6 +55,8 @@ modules-Linux:
 	mkdir -p $(sort $(dir $(zfs-objs) $(zfs-)))
 	$(MAKE) -C @LINUX_OBJ@ $(if @KERNEL_CC@,CC=@KERNEL_CC@) \
 		$(if @KERNEL_LD@,LD=@KERNEL_LD@) $(if @KERNEL_LLVM@,LLVM=@KERNEL_LLVM@) \
+		$(if @KERNEL_CROSS_COMPILE@,CROSS_COMPILE=@KERNEL_CROSS_COMPILE@) \
+		$(if @KERNEL_ARCH@,ARCH=@KERNEL_ARCH@) \
 		M="$$PWD" @KERNEL_MAKE@ CONFIG_ZFS=m modules
 
 modules-FreeBSD: