tsan: more careful handling of signals

On some tests we see that signals are not delivered
when a thread is blocked in epoll_wait. The hypothesis
is that the signal is delivered right before epoll_wait
call. The signal is queued as in_blocking_func is not set
yet, and then the thread just blocks in epoll_wait forever.
So double check pending signals *after* setting
in_blocking_func. This way we either queue a signal
and handle it in the beginning of a blocking func,
or process the signal synchronously if it's delivered
when in_blocking_func is set.

llvm-svn: 218070