page_hash_latch: Only use the spinlock implementation on
SUX_LOCK_GENERIC platforms (those for which we do not implement
a futex-like interface). Use srw_spin_mutex on 32-bit systems
(except Microsoft Windows) to satisfy the size constraints.

rw_lock::is_read_locked(): Remove. We will use the slightly
broader assertion is_locked().

srw_lock_: Implement is_locked(), is_write_locked() in a hacky
way for the Microsoft Windows SRWLOCK. This should be acceptable,
because we are only using these predicates in debug assertions
(or later, in lock elision), and false positives should not matter.