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/runtime_nintendoswitch.go

@@ -245,17 +245,17 @@ var bssStartSymbol [0]byte
 //go:extern __bss_end
 var bssEndSymbol [0]byte
 
-// Mark global variables.
+// Find global variables.
 // The linker script provides __*_start and __*_end symbols that can be used to
 // scan the given sections. They are already aligned so don't need to be
 // manually aligned here.
-func markGlobals() {
+func findGlobals(found func(start, end uintptr)) {
 	dataStart := uintptr(unsafe.Pointer(&dataStartSymbol))
 	dataEnd := uintptr(unsafe.Pointer(&dataEndSymbol))
-	markRoots(dataStart, dataEnd)
+	found(dataStart, dataEnd)
 	bssStart := uintptr(unsafe.Pointer(&bssStartSymbol))
 	bssEnd := uintptr(unsafe.Pointer(&bssEndSymbol))
-	markRoots(bssStart, bssEnd)
+	found(bssStart, bssEnd)
 }
 
 // getContextPtr returns the hblauncher context