CHROMIUM: kernelconfig sets CROSS_COMPILE

Without this, when rebuilding kernel config for aarch64 in
chromeos-kernel-upstream, as-version.sh is run with the following args:

    as-version.sh aarch64-cros-linux-gnu-clang -no-integrated-as \
        --prefix=/usr/bin/ -Werror=unknown-warning-option

and the following line fails:

    set -- $(LC_ALL=C "$@" -Wa,--version -c -x assembler /dev/null \
        -o /dev/null 2>/dev/null)

causing this error: "... unknown assembler invoked"

This is because /usr/bin/as is invoked (instead of the cross compile
version) because --prefix=/usr/bin is specified (as opposed to the
correct "--prefix=/usr/bin/${CROSS_COMPILE}")

This all started in commit ba64beb ("kbuild: check the minimum
assembler version in Kconfig") which landed in 5.12

Note: this was submitted as https://crrev.com/c/2993714 by accident
wihout running style checks.

BUG=b:190342046
TEST=run chromeos/scripts/kernelconfig olddefconfig

Signed-off-by: Eizan Miyamoto <eizan@chromium.org>
Change-Id: Ib09150dd8239bd8d139956b9ed0806a940ee1120
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/3026231
Reviewed-by: Lukasz Majczak <lmajczak@google.com>
Reviewed-by: Sonny Rao <sonnyrao@chromium.org>

chromeos/scripts/kernelconfig

@@ -83,7 +83,8 @@ build_one() {
 	set -- \
 		LD="ld.lld" \
 		CC="${ccompiler}" \
-		CXX="${cross}-g++"
+		CXX="${cross}-g++" \
+		CROSS_COMPILE="${cross}-"
 
 	local O="$(pwd)/build/${arch}"
 	mkdir -p "${O}"