runtime: allow use of >512 MB on 32-bit platforms

runtime: memory allocated by OS not in usable range
runtime: out of memory: cannot allocate 1114112-byte block (2138832896 in use)
throw: out of memory

runtime.throw+0x40 /Users/rsc/g/go/src/pkg/runtime/runtime.c:102
        runtime.throw(0x1fffd, 0x101)
runtime.mallocgc+0x2af /Users/rsc/g/go/src/pkg/runtime/malloc.c:60
        runtime.mallocgc(0x100004, 0x0, 0x1, 0x1, 0xc093, ...)
runtime.mal+0x40 /Users/rsc/g/go/src/pkg/runtime/malloc.c:289
        runtime.mal(0x100004, 0x20bc4)
runtime.new+0x26 /Users/rsc/g/go/src/pkg/runtime/malloc.c:296
        runtime.new(0x100004, 0x8fe84000, 0x20bc4)
main.main+0x29 /Users/rsc/x.go:11
        main.main()
runtime.mainstart+0xf /Users/rsc/g/go/src/pkg/runtime/386/asm.s:93
        runtime.mainstart()
runtime.goexit /Users/rsc/g/go/src/pkg/runtime/proc.c:178
        runtime.goexit()
----- goroutine created by -----
_rt0_386+0xbf /Users/rsc/g/go/src/pkg/runtime/386/asm.s:80

R=iant, r
CC=golang-dev
https://golang.org/cl/4444073

src/pkg/runtime/malloc.goc

@@ -346,7 +346,7 @@ runtime·MHeap_SysAlloc(MHeap *h, uintptr n)
 		return nil;
 
 	if(p < h->arena_start || p+n - h->arena_start >= MaxArena32) {
-		runtime·printf("runtime: memory allocated by OS not in usable range");
+		runtime·printf("runtime: memory allocated by OS not in usable range\n");
 		runtime·SysFree(p, n);
 		return nil;
 	}

src/pkg/runtime/mheap.c

@@ -180,11 +180,7 @@ MHeap_Grow(MHeap *h, uintptr npage)
 	// Allocate a multiple of 64kB (16 pages).
 	npage = (npage+15)&~15;
 	ask = npage<<PageShift;
-	if(ask > h->arena_end - h->arena_used) {
-		runtime·printf("runtime: out of memory: no room in arena for %D-byte allocation (%D in use)\n", (uint64)ask, (uint64)(h->arena_used - h->arena_start));
-		return false;
-	}
-	if(ask < HeapAllocChunk && HeapAllocChunk <= h->arena_end - h->arena_used)
+	if(ask < HeapAllocChunk)
 		ask = HeapAllocChunk;
 
 	v = runtime·MHeap_SysAlloc(h, ask);
@@ -194,7 +190,7 @@ MHeap_Grow(MHeap *h, uintptr npage)
 			v = runtime·MHeap_SysAlloc(h, ask);
 		}
 		if(v == nil) {
-			runtime·printf("runtime: out of memory: operating system refused %D-byte allocation\n", (uint64)ask);
+			runtime·printf("runtime: out of memory: cannot allocate %D-byte block (%D in use)\n", (uint64)ask, mstats.heap_sys);
 			return false;
 		}
 	}