runtime/cgo: add TSAN annotations for C sigaction call

runtime/cgo: add TSAN annotations for C sigaction call

This avoids false-positive TSAN reports when using the C sigaction
function to read handlers registered by the Go runtime.

(Unfortunately, I can't seem to coax the runtime into reproducing the
failure in a small unit-test.)

Change-Id: I744279a163708e24b1fbe296ca691935c394b5f3
Reviewed-on: https://go-review.googlesource.com/44270
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>