In case gdbsrv poll syscall fails, produces more information
gdbsrv poll syscall seems to very infrequently (1 on 100000 vgdb invocations§)
to be EINTR-upted.
So, when poll syscall fails, output stacktrace + sigmask status
to capture more info about the problem.
This is a follow-up/extension of r13748, which showed poll was interrupted
but it is not clear why. In particular, all async signals are supposed
to be masked at the time vgdb has forced an invocation



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13953 a5019735-40e9-0310-863c-91ae7b9d1cf9