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_blocks.go

@@ -37,6 +37,7 @@ import (
 )
 
 const gcDebug = false
+const needsStaticHeap = true
 
 // Some globals + constants for the entire GC.