1. Fix a data race (g_interrupt_sent flag usage was not thread safe, signals
can be handled on arbitrary threads)
2. exit() is not signal-safe, replaced it with the signal-safe equivalent
_exit()
(This differs from the patch on Phabrictor because I had to add
`#include <atomic>` to get the definition of `std::atomic_flag`.)
patch by lemo
Differential Revision: https://reviews.llvm.org/D37926
llvm-svn: 313785
6c84ffbf5f