Make sanitizer markapexes mutators parallel

The sanitizer markapexes mutators only visit dependencies and don't
modify them, mark it as able to run in parallel.

Test: all soong tests
Flag: EXEMPT refactor
Change-Id: Ie45c212e6a0c9dbc916e52777febaebece9c155c
diff --git a/cc/sanitize.go b/cc/sanitize.go
index a8722a0..094c36a 100644
--- a/cc/sanitize.go
+++ b/cc/sanitize.go
@@ -176,7 +176,7 @@
 	switch t {
 	case cfi, Hwasan, Asan, tsan, Fuzzer, scs, Memtag_stack:
 		sanitizer := &sanitizerSplitMutator{t}
-		ctx.BottomUp(t.variationName()+"_markapexes", sanitizer.markSanitizableApexesMutator)
+		ctx.BottomUp(t.variationName()+"_markapexes", sanitizer.markSanitizableApexesMutator).Parallel()
 		ctx.Transition(t.variationName(), sanitizer)
 	case Memtag_heap, Memtag_globals, intOverflow:
 		// do nothing