coreos-kernel.eclass: Install host arch Makefiles into source dir

Gentoo's linux-info.eclass tries to check the kernel version by
including /lib/modules/.../source/Makefile and printing $(VERSION).  It
unsets ARCH= before doing this (no clue why) which works with a full
source tree but not with our minimized one. To fix this we need to
archive arch/x86/Makefile also for arm64.

Signed-off-by: Jeremi Piotrowski <jpiotrowski@microsoft.com>

sdk_container/src/third_party/coreos-overlay/eclass/coreos-kernel.eclass

@@ -187,6 +187,7 @@ shred_keys() {
 # Populate /lib/modules/$(uname -r)/{build,source}
 install_build_source() {
 	local kernel_arch=$(tc-arch-kernel)
+	local host_kernel_arch=$(tc-ninja_magic_to_arch kern "${CBUILD}")
 
 	# NOTE: We have to get ${archabspaths} before removing symlinks under
 	# /usr/lib/modules. However, do not exclude "dt-bindings" for now,
@@ -232,6 +233,7 @@ install_build_source() {
 	# or arm64.
 	{
 		echo source/Makefile
+		find source/arch/${host_kernel_arch} -follow -maxdepth 1 -name 'Makefile*' -print
 		find source/arch/${kernel_arch} -follow -maxdepth 1 -name 'Makefile*' -print
 		find source/arch/${kernel_arch} -follow \( -name 'module.lds' -o -name 'Kbuild.platforms' -o -name 'Platform' \) -print
 		find $(find source/arch/${kernel_arch} -follow \( -name include -o -name scripts \) -follow -type d -print) -print