xen/bitops: Cleanup and new infrastructure ahead of rearrangements

xen/bitops: Cleanup and new infrastructure ahead of rearrangements

 * Rename __attribute_pure__ to just __pure before it gains users.
 * Introduce __constructor which is going to be used in lib/, and is
   unconditionally cf_check.
 * Identify the areas of xen/bitops.h which are a mess.
 * Introduce xen/self-tests.h as helpers for compile and boot time testing.
   This provides a statement of the ABI, and a confirmation that arch-specific
   implementations behave as expected.
 * Introduce HIDE() in macros.h.  While it's only used in self-tests.h for
   now, we're going to consolidate similar constructs in due course.

Sadly Clang 7 and older isn't happy with the compile time checks.  Skip them,
and just rely on the runtime checks.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>