xen/string: Clean up {xen,arm}/string.h

xen/string: Clean up {xen,arm}/string.h

 * Drop __kernel_size_t entirely.  It isn't a useful distinction, especially
   as it means the the prototypes don't appear to match their common
   definitions.
 * Introduce __HAVE_ARCH_* guards for strpbrk(), strsep() and strspn(), which
   match their implementation in common/string.c
 * Apply consistent Xen style throughout.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <julien.grall@arm.com>