We might be forwarding to a C signal handler.  C code expects the stack
to be aligned.  Should fix darwin/386 build: the testcarchive tests were
hanging as the program got an endless series of SIGSEGV signals.

Change-Id: Ia02485d3736a3c40e12259f02d25f842cf8e4d29
Reviewed-on: https://go-review.googlesource.com/19025
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>