runtime/cgo: use libc for sigaction syscalls when possible

runtime/cgo: use libc for sigaction syscalls when possible

This ensures that runtime's signal handlers pass through the TSAN and
MSAN libc interceptors and subsequent calls to the intercepted
sigaction function from C will correctly see them.

Fixes #17753.

Change-Id: I9798bb50291a4b8fa20caa39c02a4465ec40bb8d
Reviewed-on: https://go-review.googlesource.com/33142
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>