xen/config.h: Move BITS_PER_* definitions from asm/config.h to xen/config.h

xen/config.h: Move BITS_PER_* definitions from asm/config.h to xen/config.h

BITS_PER_* values can be defined in a common way using compiler-provided macros.
Thus, these definitions are moved to xen/config.h to reduce duplication across
architectures.

Additionally, *_BYTEORDER macros are removed, as BITS_PER_* values now come
directly from the compiler environment.

The arch_fls() implementation for Arm and PPC is updated to use BITS_PER_INT
instead of a hardcoded value of 32.

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com>