Add new headers_check_all target for checking all arches in one go.

Useful for distros (and people with too much time on their hands) that support
a ton of architectures, headers_check_all is to headers_check as
headers_install_all is to headers_install
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>