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_globals.go

@@ -2,13 +2,14 @@
 
 package runtime
 
-// This file implements markGlobals for all the files that don't have a more
-// specific implementation.
+// This file implements findGlobals for all systems where the start and end of
+// the globals section can be found through linker-defined symbols.
 
-// markGlobals marks all globals, which are reachable by definition.
+// findGlobals finds all globals (which are reachable by definition) and calls
+// the callback for them.
 //
 // This implementation marks all globals conservatively and assumes it can use
 // linker-defined symbols for the start and end of the .data section.
-func markGlobals() {
-	markRoots(globalsStart, globalsEnd)
+func findGlobals(found func(start, end uintptr)) {
+	found(globalsStart, globalsEnd)
 }