Rewrite `Atomic_RW_Mutex`

This patch simplifies the implementation and fixes #5254.

Previously, the mutex was set up as if there could be multiple writers,
and there seemed to be some confusion as to which `Writer` bits to
check, as not all were checked or set at the same time.

This could also result in the mutex being left in a non-zero state even
after unlocking all locks.

All unneeded state has been removed and extra checks have been put in
place.

0 files changed, 0 insertions, 0 deletions