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/os_windows.go

@@ -52,7 +52,7 @@ var module *exeHeader
 // around 160 bytes of amd64 instructions.
 // Most of this function is based on the documentation in
 // https://docs.microsoft.com/en-us/windows/win32/debug/pe-format.
-func markGlobals() {
+func findGlobals(found func(start, end uintptr)) {
 	// Constants used in this function.
 	const (
 		// https://docs.microsoft.com/en-us/windows/win32/api/libloaderapi/nf-libloaderapi-getmodulehandleexa
@@ -85,7 +85,7 @@ func markGlobals() {
 			// Found a writable section. Scan the entire section for roots.
 			start := uintptr(unsafe.Pointer(module)) + uintptr(section.virtualAddress)
 			end := uintptr(unsafe.Pointer(module)) + uintptr(section.virtualAddress) + uintptr(section.virtualSize)
-			markRoots(start, end)
+			found(start, end)
 		}
 		section = (*peSection)(unsafe.Add(unsafe.Pointer(section), unsafe.Sizeof(peSection{})))
 	}