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_leaking.go

@@ -11,6 +11,8 @@ import (
 	"unsafe"
 )
 
+const needsStaticHeap = true
+
 // Ever-incrementing pointer: no memory is freed.
 var heapptr uintptr