uapi: Define GENMASK_U128

This adds GENMASK_U128() and __GENMASK_U128() macros using __BITS_PER_U128
and __int128 data types. These macros will be used in providing support for
generating 128 bit masks.

The macros wouldn't work in all assembler flavors for reasons described
in the comments on top of declarations. Enforce it for more by adding
!__ASSEMBLY__ guard.

Cc: Yury Norov <yury.norov@gmail.com>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Arnd Bergmann <arnd@arndb.de>>
Cc: linux-kernel@vger.kernel.org
Cc: linux-arch@vger.kernel.org
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Yury Norov <yury.norov@gmail.com>