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 <dhowells@redhat.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Dave Jones <davej@redhat.com>