win32: pthread: don't play dirty tricks for mutex init

We used double-checked locking on pthread_mutex_t.requires_init in order
to lazily initialize static mutexes (since CRITICAL_SECTION has no
native way to do this). This was kind of unclean: we relied on MSVC
semantics for volatile (which apparently means all accesses are weakly
atomic), which is not such a good idea since mpv can't even be compiled
with MSVC.

Since it's too much of a pain to get weak atomics, just use INIT_ONCE
for initializing the CRITICAL_SECTION. Microsoft most likely implemented
this in an extremely efficient way. Essentially, it provides a mechanism
for correct double-checked locking without having to deal with the
tricky details. We still use an extra flag to avoid calling it at all
for normal locks.

(To get weak atomics, we could have used stdatomic.h, which modern MinGW
provides just fine. But I don't want this wrapper depend on MinGW
specifics if possible.)

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