Pass "-Xgc:CMC" to dex2oat when userfaultfd GC is enabled.

This change is a no-op change. It doesn't enable userfaultfd GC by
default. ENABLE_UFFD_GC=true can be passed to the build system to enable
userfaultfd GC for testing purposes.

Bug: 242553398
Test: -
  1. lunch aosp_oriole-userdebug
  2. ENABLE_UFFD_GC=true m
  3. See "--runtime-arg -Xgc:CMC" in .invocation files.
Change-Id: I789b49a71b9604fd41bf1ef77d0ac5bac4cbdf25

java/dexpreopt_bootjars.go

@@ -752,6 +752,10 @@ func buildBootImageVariant(ctx android.ModuleContext, image *bootImageVariant, p
 		cmd.FlagWithArg("--instruction-set-features=", global.InstructionSetFeatures[arch])
 	}
 
+	if global.EnableUffdGc {
+		cmd.Flag("--runtime-arg").Flag("-Xgc:CMC")
+	}
+
 	if global.BootFlags != "" {
 		cmd.Flag(global.BootFlags)
 	}