kbuild: add --target to correctly cross-compile UAPI headers with Clang

[ Upstream commit 9fbed27a7a1101c926718dfa9b49aff1d04477b5 ]

When you compile-test UAPI headers (CONFIG_UAPI_HEADER_TEST=y) with
Clang, they are currently compiled for the host target (likely x86_64)
regardless of the given ARCH=.

In fact, some exported headers include libc headers. For example,
include/uapi/linux/agpgart.h includes <stdlib.h> after being exported.
The header search paths should match to the target we are compiling
them for.

Pick up the --target triple from KBUILD_CFLAGS in the same ways as
commit 7f58b487e9ff ("kbuild: make Clang build userprogs for target
architecture").

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Stable-dep-of: 02e9a22ceef0 ("kbuild: hdrcheck: fix cross build with clang")
Signed-off-by: Sasha Levin <sashal@kernel.org>