parisc: Add lws_atomic_xchg and lws_atomic_store syscalls

This patch adds two new LWS routines - lws_atomic_xchg and lws_atomic_store.

These are simpler than the CAS routines.  Currently, we use the CAS
routines for atomic stores.  This is inefficient since it requires
both winning the spinlock and a successful CAS operation.

Change has been tested on c8000 and rp3440.

In v2, I moved the code to disble/enable page faults inside the spinlocks.

Signed-off-by: John David Anglin <dave.anglin@bell.net>
Signed-off-by: Helge Deller <deller@gmx.de>