On Linux, OpenBSD and Microsoft Windows, srw_mutex was an alias for a
rw-lock while we only need mutex functionality. Let us implement a
futex-based mutex with one bit for HOLDER and 31 bits for counting
waiting requests.

srw_lock::wr_unlock() can avoid waking up a waiter when no waiting
requests exist. (Previously, we only had 1-bit rw_lock::WRITER_WAITING
flag that could be wrongly cleared if multiple waiting wr_lock() exist.
Now we have no problem with up to 2,147,483,648 conflicting threads.)

On 64-bit Microsoft Windows, the advantage is that
sizeof(srw_mutex) is 4, while sizeof(SRWLOCK) would be 8.

Reviewed by: Vladislav Vaintroub