Make BottomUpMutators parallel

Append .Parallel() to all of the RegisterBottomUpMutator calls to tell
Blueprint it can run them in parallel.

Test: identical build.ninja, passes race detector
Change-Id: I969a0689522d4cba7c8ff51e2aa00fe2fd338a89
diff --git a/android/arch.go b/android/arch.go
index c767bd7..5074c4c 100644
--- a/android/arch.go
+++ b/android/arch.go
@@ -25,10 +25,10 @@
 )
 
 func init() {
-	RegisterBottomUpMutator("defaults_deps", defaultsDepsMutator)
+	RegisterBottomUpMutator("defaults_deps", defaultsDepsMutator).Parallel()
 	RegisterTopDownMutator("defaults", defaultsMutator)
 
-	RegisterBottomUpMutator("arch", ArchMutator)
+	RegisterBottomUpMutator("arch", ArchMutator).Parallel()
 }
 
 var (