Fixes Darwin 386 build. It turns out that the Darwin pthread_create
function saves the SSE registers, and therefore requires an aligned stack.
This worked before https://golang.org/cl/70530 because the stack sizes
were chosen to leave the stack aligned.

Change-Id: I911a9e8dcde4e41e595d5ef9b9a1ca733e154de6
Reviewed-on: https://go-review.googlesource.com/71432Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Austin Clements <austin@google.com>