runtime: refactor GC mark phase into gcMarkReachable

This is a small refactor to prepare GC marking for multithreaded
stop-the-world.

src/runtime/gc_blocks.go

@@ -456,8 +456,7 @@ func runGC() (freeBytes uintptr) {
 	}
 
 	// Mark phase: mark all reachable objects, recursively.
-	markStack()
-	findGlobals(markRoots)
+	gcMarkReachable()
 
 	if baremetal && hasScheduler {
 		// Channel operations in interrupts may move task pointers around while we are marking.

src/runtime/gc_stack_portable.go

@@ -8,6 +8,11 @@ import (
 	"unsafe"
 )
 
+func gcMarkReachable() {
+	markStack()
+	findGlobals(markRoots)
+}
+
 //go:extern runtime.stackChainStart
 var stackChainStart *stackChainObject
 

src/runtime/gc_stack_raw.go

@@ -4,6 +4,11 @@ package runtime
 
 import "internal/task"
 
+func gcMarkReachable() {
+	markStack()
+	findGlobals(markRoots)
+}
+
 // markStack marks all root pointers found on the stack.
 //
 // This implementation is conservative and relies on the stack top (provided by