tools/nolibc: add target to check header usability

Each nolibc header should be valid for inclusion irrespective of any
special ordering requirements.

Add a new make target, based on the old kbuild "make header_check" target
to validate this requirement.

For now the check fails, but the following commits will fix the issues.

Signed-off-by: Thomas Weißschuh <[e-mail]>
Acked-by: Willy Tarreau <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

tools/include/nolibc/Makefile

···
 	$(Q)$(MAKE) -C $(srctree) headers
 	$(Q)$(MAKE) -C $(srctree) headers_install INSTALL_HDR_PATH=$(OUTPUT)sysroot
 
+# GCC uses "s390", clang "systemz"
+CLANG_CROSS_FLAGS := $(subst --target=s390-linux,--target=systemz-linux,$(CLANG_CROSS_FLAGS))
+
+headers_check: headers_standalone
+	for header in $(filter-out crt.h std.h,$(all_files)); do \
+		$(CC) $(CLANG_CROSS_FLAGS) -Wall -Werror -nostdinc -fsyntax-only -x c /dev/null \
+			-I$(or $(objtree),$(srctree))/usr/include -include $$header -include $$header || exit 1; \
+	done
+
 clean:
 	$(call QUIET_CLEAN, nolibc) rm -rf "$(OUTPUT)sysroot"