Otherwise, we won't get a stack trace in some of the early init.

Here's one example:

        http://build.golang.org/log/a96d10f6aee1fa3e3ae51f41da46d414a7ab02de

After walking the stack by hand in acid, I was able to determine
that the stackalloc inside mpreinit was causing the throw.

LGTM=rsc
R=rsc, dvyukov
CC=golang-codereviews
https://golang.org/cl/72450044