gcc-plugins: Force full rebuild when plugins change

There was no dependency between the plugins changing and the rest of the
kernel being built. This could cause strange behaviors as instrumentation
could vary between targets depending on when they were built.

Generate a new header file, gcc-plugins.h, any time the GCC plugins
change. Include the header file in compiler-version.h when its associated
feature name, GCC_PLUGINS, is defined. This will be picked up by fixdep
and force rebuilds where needed.

Add a generic "touch" kbuild command, which will be used again in
a following patch. Add a "normalize_path" string helper to make the
"TOUCH" output less ugly.

Link: https://lore.kernel.org/r/20250503184623.2572355-1-kees@kernel.org


Tested-by: Nicolas Schier <n.schier@avm.de>
Reviewed-by: Nicolas Schier <n.schier@avm.de>
Signed-off-by: Kees Cook <kees@kernel.org>