gh-149879: Fix test_signal on Cygwin (#149896)

* Check for SIG_BLOCK instead of pthread_sigmask() to decide if
  SIG_BLOCK, SIG_UNBLOCK and SIG_SETMASK constants should be
  converted to enums.
* Skip ITIMER_VIRTUAL and ITIMER_PROF tests on Cygwin: setitimer()
  fails with ItimerError(EINVAL).