kbuild: make Clang build userprogs for target architecture

Programs added 'userprogs' should be compiled for the target
architecture i.e. the same architecture as the kernel.

GCC does this correctly since the target architecture is implied
by the toolchain prefix.

Clang builds userspace programs always for the host architecture
because the target triple is currently missing.

Fix this.

Fixes: 7f3a59db ("kbuild: add infrastructure to build userspace programs")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>

Makefile

@@ -970,8 +970,8 @@ LDFLAGS_vmlinux	+= --pack-dyn-relocs=relr
 endif
 
 # Align the bit size of userspace programs with the kernel
-KBUILD_USERCFLAGS  += $(filter -m32 -m64, $(KBUILD_CFLAGS))
-KBUILD_USERLDFLAGS += $(filter -m32 -m64, $(KBUILD_CFLAGS))
+KBUILD_USERCFLAGS  += $(filter -m32 -m64 --target=%, $(KBUILD_CFLAGS))
+KBUILD_USERLDFLAGS += $(filter -m32 -m64 --target=%, $(KBUILD_CFLAGS))
 
 # make the checker run with the right architecture
 CHECKFLAGS += --arch=$(ARCH)