mutex: save power with better cpu_relax
Use cpu_relaxed_read and cpu_read_relax to allow more architectures to be able
to "relax" instead of busy spinning.

Bug 1440421

Signed-off-by: Alex Van Brunt <avanbrunt@nvidia.com>
Reviewed-by: Sumit Singh <sumsingh@nvidia.com>
Build v3.98.1 EXP2