runtime/cgo: fixes for calling sigaction in C

runtime/cgo: fixes for calling sigaction in C

Zero out the sigaction structs, in case the sa_restorer field is set.

Clear the SA_RESTORER flag; it is part of the kernel interface, not the
libc interface.

Fixes #17947.

Change-Id: I610348ce3c196d3761cf2170f06c24ecc3507cf7
Reviewed-on: https://go-review.googlesource.com/33331
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>