libgo: support x32 as GOARCH=amd64p32 GOOS=linux

This is enough to let libgo build when configured using
--with-multilib-list=m64,m32,mx32.  I don't have an x32-enabled kernel
so I haven't tested whether it executes correctly.

For https://gcc.gnu.org/PR87470

Change-Id: Ie174e828b6ca6294de98048047681dd9e1b99422
Reviewed-on: https://go-review.googlesource.com/138817
Reviewed-by: Than McIntosh <thanm@google.com>
6 files changed