The fwdSig array is accessed by the signal handler, which may run in
parallel with other threads manipulating it via the os/signal package.
Use atomic accesses to ensure that there are no problems.

Move the _SigHandling flag out of the sigtable array. This makes sigtable
immutable and safe to read from the signal handler.

Change-Id: Icfa407518c4ebe1da38580920ced764898dfc9ad
Reviewed-on: https://go-review.googlesource.com/36321
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>