debuggerd: add SIGSYS to the list of handled signals.

Processes using seccomp will get SIGSYS when attempting to use a
disallowed syscall. We're currently not handling SIGSYS in bionic's
signal handler, but this change will let us dump a backtrace pointing at
rt_tgsigqueueinfo in our signal handler if seccomp policies block it
during a real crash.

Bug: http://b/27853687
Change-Id: I4e4aacc95eeef7249d895e19dc4ccd77a51c7a17