kbuild: uapi: use custom stub headers instead of libc ones

Now that custom stub headers are provided for all libc headers used from
the UAPI headers, the dependency on the toolchain libc can be dropped.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Tested-by: Nicolas Schier <nsc@kernel.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://patch.msgid.link/20260227-kbuild-uapi-libc-v1-7-c17de0d19776@weissschuh.net
Signed-off-by: Nicolas Schier <nsc@kernel.org>

usr/include/Makefile

@@ -138,14 +138,13 @@ endif
 always-y := $(patsubst $(obj)/%.h,%.hdrtest, $(shell find $(obj) -name '*.h' 2>/dev/null))
 
 target-no-libc = $(filter-out $(uses-libc), $*.h)
-target-can-compile = $(and $(filter-out $(no-header-test), $*.h), \
-                           $(or $(CONFIG_CC_CAN_LINK), $(target-no-libc)))
+target-can-compile = $(filter-out $(no-header-test), $*.h)
 
 # Include the header twice to detect missing include guard.
 quiet_cmd_hdrtest = HDRTEST $<
       cmd_hdrtest = \
 		$(CC) $(c_flags) -fsyntax-only -Werror -x c /dev/null \
-			$(if $(target-no-libc), -nostdinc, -I $(srctree)/usr/dummy-include) \
+			-nostdinc $(if $(target-no-libc), , -I $(srctree)/usr/dummy-include) \
 			$(if $(target-can-compile), -include $< -include $<); \
 		$(PERL) $(src)/headers_check.pl $(obj) $<; \
 		touch $@