kbuild: uapi: reuse KBUILD_USERCFLAGS

The toplevel Makefile already provides the compiler flags necessary to
build userspace applications for the target.

Make use of them instead of duplicating the logic.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://patch.msgid.link/20251014-kbuild-uapi-usercflags-v1-1-c162f9059c47@linutronix.de
Signed-off-by: Nicolas Schier <nsc@kernel.org>

Author: t-8ch

usr/include/Makefile

@@ -6,16 +6,9 @@
 #
 # -std=c90 (equivalent to -ansi) catches the violation of those.
 # We cannot go as far as adding -Wpedantic since it emits too many warnings.
-UAPI_CFLAGS := -std=c90 -Wall -Werror=implicit-function-declaration
+UAPI_CFLAGS := -std=c90 -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 --target=%, $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS))
-
-# USERCFLAGS might contain sysroot location for CC.
-UAPI_CFLAGS += $(USERCFLAGS)
-
-override c_flags = $(UAPI_CFLAGS) -Wp,-MMD,$(depfile) -I $(obj) -I $(srctree)/usr/dummy-include
+override c_flags = $(KBUILD_USERCFLAGS) $(UAPI_CFLAGS) -Wp,-MMD,$(depfile) -I $(obj) -I $(srctree)/usr/dummy-include
 
 # The following are excluded for now because they fail to build.
 #