tools/nolibc: add target to check header usability

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 <thomas.weissschuh@linutronix.de>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20250424-nolibc-header-check-v1-1-011576b6ed6f@linutronix.de