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_linux.go

@@ -77,9 +77,9 @@ type elfProgramHeader32 struct {
 //go:extern __ehdr_start
 var ehdr_start elfHeader
 
-// markGlobals marks all globals, which are reachable by definition.
+// findGlobals finds globals in the .data/.bss sections.
 // It parses the ELF program header to find writable segments.
-func markGlobals() {
+func findGlobals(found func(start, end uintptr)) {
 	// Relevant constants from the ELF specification.
 	// See: https://refspecs.linuxfoundation.org/elf/elf.pdf
 	const (
@@ -99,14 +99,14 @@ func markGlobals() {
 			if header._type == PT_LOAD && header.flags&PF_W != 0 {
 				start := header.vaddr
 				end := start + header.memsz
-				markRoots(start, end)
+				found(start, end)
 			}
 		} else {
 			header := (*elfProgramHeader32)(headerPtr)
 			if header._type == PT_LOAD && header.flags&PF_W != 0 {
 				start := header.vaddr
 				end := start + header.memsz
-				markRoots(start, end)
+				found(start, end)
 			}
 		}
 		headerPtr = unsafe.Add(headerPtr, ehdr_start.phentsize)