[sanitizer] Fix deadlock in StopTheWorld.

The tracer thread in StopTheWorld could wait on a mutex forever if the parent
process died before unlocking it. Use PR_SET_PDEATHSIG so that the parent would
kill the child in this scenario.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@192210 91177308-0d34-0410-b5e6-96231b3b80d8