Clear read_fd_set if EINTR received

Leaving bits uncleared set causes callbacks to be triggered even
though there are no events to process. Starting with D131160
we have a callback that makes blocking read calls over pipe which
was causing the lldb-server main loop to become unresponsive / blocked
on Android.

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D144240