Fix the SIGINT handlers

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