xen/arm: bitops: Implement a new set of helpers that can timeout

xen/arm: bitops: Implement a new set of helpers that can timeout

Exclusive load-store atomics should only be used between trusted
threads. As not all the guests are trusted, it may be possible to DoS
Xen when updating shared memory with guest atomically.

To prevent the infinite loop, we introduce a new set of helpers that can
timeout. The timeout is based on the maximum number of iterations.

They will be used in follow-up patch to make atomic operations
on shared memory safe.

This is part of XSA-295.

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