winpthreads initializes a statically initialized rwlock on its first
lock and fails a concurrent first lock with EINVAL, which
std::shared_mutex ignores, so two threads can hold the lock at once.
The source, binding plan and specialized module caches take their
first locks from several threads, so their mutexes now take and
release the lock in their constructor, as guest rwlocks do.
The GuestWriteWatch mutex the technical debt entry also named is in
the Linux-only watch, so it is not affected.
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