runtime: refactor markGlobals to findGlobals

Instead of markGlobals calling markRoots unconditionally (which doesn't
make sense for -gc=none and -gc=leaking), provide markRoots as a
callback function.

This is in preparation for -gc=boehm, where the previous design is even
more awkward and a callback makes far more sense.

I've tested the size impact using `make smoketest XTENSA=0`. There is
none, except for two cases:

  * One with `-opt=0` so const-propagation for the callback didn't take
    place.
  * One other on AVR, I don't know why but as it's only 16 bytes in a
    very specific case I'm going to assume it's just a random change in
    compiler output that caused a size difference.

src/runtime/gc_none.go

@@ -37,7 +37,3 @@ func initHeap() {
 func setHeapEnd(newHeapEnd uintptr) {
 	// Nothing to do here, this function is never actually called.
 }
-
-func markRoots(start, end uintptr) {
-	// dummy, so that markGlobals will compile
-}