runtime: release worldsema before Gosched in STW GC mode
After CL 182657 we no longer hold worldsema across the GC, we hold
gcsema instead.
However in STW GC mode we don't release worldsema before calling Gosched
on the user goroutine (note that user goroutines are disabled during STW
GC) so that user goroutine holds onto it. When the GC is done and the
runtime inevitably wants to "stop the world" again (though there isn't
much to stop) it'll sit there waiting for worldsema which won't be
released until the aforementioned goroutine is scheduled, which it won't
be until the GC is done!
So, we have a deadlock.
The fix is easy: just release worldsema before calling Gosched.