GCC will insert various calls to pthread functions when compiling
C++ code with static initializers, even when the user doesn't link
their program against libpthread explicitly.
This is used to make static initializers thread-safe, e.g. when
building a library that does not itself use thread functionality
and thus does not link against libpthread - but is intended to
be used with other code that does use libpthread explicitly.
This makes these symbols available in libc.
This required a bit of rearchitecture, as pthread_atfork() required a
mutex, and duplicating a mutex impl for it was silly.
As such, this patch moves some standalone bits of pthread into LibC and
uses those to implement atfork().
It should be noted that for programs that don't use atfork(), this
mechanism only costs two atomic loads (as opposed to the normal mutex
lock+unlock) :^)
