[PATCH] Run the callback on a separate stack in StopTheWorld.

[Alexander Potapenko]

  LGTM.
  It turns out that we must always clean up the excessive 120 bytes of sigset_t, because each call to sigaction() may store random stack variables into *oldact.
  I suggest you to send a separate CL with a global sigaction wrapper that cleans up *oldact (at least let's have a bug report about that).
  We'll also need to change the sigaction() call to internal_sigaction() if we want this to work under ASan.

http://llvm-reviews.chandlerc.com/D592