tsan: fix signal handling during stop-the-world

Long story short: stop-the-world briefly resets SIGSEGV handler to SIG_DFL.
This breaks programs that handle and continue after SIGSEGV (namely JVM).
See the test and comments for details.

This is reincarnation of reverted r229678 (http://reviews.llvm.org/D7722).
Changed:
- execute TracerThreadDieCallback only on tracer thread
- reset global data in TracerThreadSignalHandler/TracerThreadDieCallback
- handle EINTR from waitpid

Add 3 new test:
- SIGSEGV during leak checking
- StopTheWorld operation during signal storm from an external process
- StopTheWorld operation when the program generates and handles SIGSEGVs

http://reviews.llvm.org/D8032

llvm-svn: 231367