runtime: add "end of heap" to detect out of memory

This can be used in the future to trigger garbage collection. For now,
it provides a more useful error message in case the heap is completely
filled up.

src/runtime/gc_dumb.go

@@ -20,6 +20,9 @@ func alloc(size uintptr) unsafe.Pointer {
 	size = align(size)
 	addr := heapptr
 	heapptr += size
+	if heapptr >= heapEnd {
+		runtimePanic("out of memory")
+	}
 	for i := uintptr(0); i < uintptr(size); i += 4 {
 		ptr := (*uint32)(unsafe.Pointer(addr + i))
 		*ptr = 0