[Signal] Re-raise SIGPIPE if the handler is uninstalled

Instead of falling through to RunSignalHandlers after the SIGPIPE
handler is uninstalled and we get a SIGPIPE, re-raise the signal, just
like we do for other IntSigs.

This was discussed and informally OK'd here:

https://reviews.llvm.org/rG9a3f892d018238dce5181e458905311db8e682f5#856804