Set up uapi/asm/Kbuild.asm. This requires the mandatory headers to be
dynamically detected. The same goes for include/asm/Kbuild.asm. The problem
is that the header files will be split or moved one at a time, but each header
file in Kbuild.asm's list applies to all arch headers of that name
simultaneously.

The dynamic detection of mandatory files can be undone later.

Signed-off-by: David Howells
Acked-by: Arnd Bergmann
Acked-by: Thomas Gleixner
Acked-by: Paul E. McKenney
Acked-by: Dave Jones