Remove incorrect check for sa_mask in signal chaining

This removes an incorrect check using the sa_mask field of the
sigaction structure when chaining to a user's signal handler.
The check prevented a user's handler being called when sa_mask
was set.

Thanks to primiano@ for the excellent bug report.

Bug: 16005022
Change-Id: I0548003f4fc3b1889a6859091e603ead4a9b0607