runtime: signal forwarding

Forward signals to signal handlers installed before Go installs its own,
under certain circumstances.  In particular, as iant@ suggests, signals are
forwarded iff:
   (1) a non-SIG_DFL signal handler existed before Go, and
   (2) signal is synchronous (i.e., one of SIGSEGV, SIGBUS, SIGFPE), and
   	(3a) signal occured on a non-Go thread, or
   	(3b) signal occurred on a Go thread but in CGo code.

Supported only on Linux, for now.

Change-Id: I403219ee47b26cf65da819fb86cf1ec04d3e25f5
Reviewed-on: https://go-review.googlesource.com/8712


Reviewed-by: Ian Lance Taylor <iant@golang.org>