runtime: fallback to 128M address space on 32bit

Available darwin/arm devices sporadically have trouble mapping 256M.

I would really appreciate it if anyone could check my working on
this, and make sure sure there aren't obviously bad consequences I
haven't considered.

Change-Id: Id1a8edae104d974fcf5f9333274f958625467f79
Reviewed-on: https://go-review.googlesource.com/5752Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/malloc.go

@@ -292,6 +292,7 @@ func mallocinit() {
 		arenaSizes := []uintptr{
 			512 << 20,
 			256 << 20,
+			128 << 20,
 		}
 
 		for _, arenaSize := range arenaSizes {