A typo in the previous revision ("act" instead of "oldact") caused us
to return the sa_flags from the new (or zeroed) sigaction rather than
the old one.

In the presence of a signal handler registered before
runtime.libpreinit, this caused setsigstack to erroneously zero out
important sa_flags (such as SA_SIGINFO) in its attempt to re-register
the existing handler with SA_ONSTACK.

Change-Id: I3cd5152a38ec0d44ae611f183bc1651d65b8a115
Reviewed-on: https://go-review.googlesource.com/37852
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>