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_blocks.go

@@ -420,7 +420,7 @@ func runGC() (freeBytes uintptr) {
 
 	// Mark phase: mark all reachable objects, recursively.
 	markStack()
-	markGlobals()
+	findGlobals(markRoots)
 
 	if baremetal && hasScheduler {
 		// Channel operations in interrupts may move task pointers around while we are marking.