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_custom.go

@@ -36,6 +36,8 @@ import (
 	"unsafe"
 )
 
+const needsStaticHeap = false
+
 // initHeap is called when the heap is first initialized at program start.
 func initHeap()