runtime: 8-byte align the heap_live field for atomic access

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>

src/runtime/mstats.go

@@ -100,6 +100,8 @@ type mstats struct {
 	// must be complete.
 	gc_trigger uint64
 
+	_ uint32 // force 8-byte alignment of heap_live and prevent an alignment check crash on MIPS32.
+
 	// heap_live is the number of bytes considered live by the GC.
 	// That is: retained by the most recent GC plus allocated
 	// since then. heap_live <= heap_alloc, since heap_alloc
@@ -411,6 +413,11 @@ func init() {
 		println(sizeof_C_MStats, unsafe.Sizeof(memStats))
 		throw("MStats vs MemStatsType size mismatch")
 	}
+
+	if unsafe.Offsetof(memstats.heap_live)%8 != 0 {
+		println(unsafe.Offsetof(memstats.heap_live))
+		throw("memstats.heap_live not aligned to 8 bytes")
+	}
 }
 
 // ReadMemStats populates m with memory allocator statistics.