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

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 7f58b48 ("kbuild: make Clang build userprogs for target
architecture").

Signed-off-by: Masahiro Yamada <[email protected]>
Reviewed-by: Nathan Chancellor <[email protected]>
Reviewed-by: Nick Desaulniers <[email protected]>

usr/include/Makefile

@@ -10,7 +10,7 @@ UAPI_CFLAGS := -std=c90 -Wall -Werror=implicit-function-declaration
 
 # In theory, we do not care -m32 or -m64 for header compile tests.
 # It is here just because CONFIG_CC_CAN_LINK is tested with -m32 or -m64.
-UAPI_CFLAGS += $(filter -m32 -m64, $(KBUILD_CFLAGS))
+UAPI_CFLAGS += $(filter -m32 -m64 --target=%, $(KBUILD_CFLAGS))
 
 # USERCFLAGS might contain sysroot location for CC.
 UAPI_CFLAGS += $(USERCFLAGS)