sigfwd calls an arbitrary C signal handler function.  The System V ABI
for x86_64 (and the most recent revision of the ABI for i386) requires
the stack to be 16-byte aligned.

Fixes: #17641

Change-Id: I77f53d4a8c29c1b0fe8cfbcc8d5381c4e6f75a6b
Reviewed-on: https://go-review.googlesource.com/32107
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>