ck_rwlock: Migrate to 32-bit primitives and add a write_latch operation.

write_latch allows writers to prevent incoming read acquisitions from
succeeding while permitting write_lock operations to complete. This is
useful if multiple writers are cooperating in linearizing a non-blocking
data structure and allows for lock state to be copied by the write lock
holder safely.

In collaboration with Paul Khuong and Jaidev Sridhar.
Merge branch 'master' of git.concurrencykit.org:ck