mheap_.heap_live is an atomically accessed uint64. It is currently not 8-byte
aligned on 32-bit platforms, which has been okay because it's only accessed via
Xadd64, which doesn't require alignment on 386 or ARM32. However, Xadd64 on
MIPS32 does require 8-byte alignment.

Add a padding field to force 8-byte alignment of heap_live and prevent an
alignment check crash on MIPS32.

Change-Id: I7eddf7883aec7a0a7e0525af5d58ed4338a401d0
Reviewed-on: https://go-review.googlesource.com/31635
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>