kbuild: in headers_install autoconvert asm/inline/volatile to __xxx__

Headers in userspace should be using the __xxx__ form of the asm, inline,
and volatile keywords.  Since people like to revert these things without
realizing what's going on, have the headers install step autoconvert these
keywords.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>