Try to synchronize threads initializations

[yuyichao]

Using uv_barrier_wait as @tkelman suggested in #14091 (comment) . I've also thought about ti_threadgroup_barrier but I'm not sure if it is available at this point (without introducing more race conditions)....

Tested on top of #14083 and it seems to fix the segfault for me.

Fixes #14091

[tkelman]

I rebased this on top of #14083 locally, and with LLVM 3.3, threading enabled, in a mingw build (either 32 or 64 bit), it changes the situation from a racy segfault to a repeatable failure in bootstrap:

This application has requested the Runtime to terminate it in an unusual way.
Please contact the application's support team for more information.

Having trouble getting a backtrace of where this is happening. In gdb I can step through initialization and it freezes in ntdll!ExpInterlockedPopEntrySListEnd16 during uv_signals_init.

[yuyichao]

Close in favor of #12904 (assuming it doesn't take too long to sort out).