xen/const: Extend the existing macro BIT to take a suffix in parameter

xen/const: Extend the existing macro BIT to take a suffix in parameter

Arm currently provides two macro BIT and BIT_ULL that are only usable
in C and return respectively unsigned long and unsigned long long.

Extending the macros to deal with assembly would be a nice benefits as
it could replace the common pattern to define fields (AC(1, sfx) << X)
easier to read.

Rather than extending the two macros, it was decided to drop BIT_ULL()
and extend the macro BIT() to take a suffix (e.g U, UL, ULL) in
parameter. This would allow to use different suffix without having to
define new macros.

The new extend macro is now moved in include/xen/const.h so it can be
used by anyone in Xen and also avoid to include bitops.h in assembly
code.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>