runtime: only allocate heap memory when needed

For example, with -gc=none and -gc=leaking, no heap needs to be
allocated when initializing the runtime. And some GCs (like -gc=custom)
are responsible for allocating the heap themselves.

src/runtime/gc_none.go

@@ -10,6 +10,8 @@ import (
 	"unsafe"
 )
 
+const needsStaticHeap = false
+
 var gcTotalAlloc uint64 // for runtime.MemStats
 var gcMallocs uint64
 var gcFrees uint64