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
diff --git a/dexpreopt/config.go b/dexpreopt/config.go
index eefda19..609a29c 100644
--- a/dexpreopt/config.go
+++ b/dexpreopt/config.go
@@ -96,6 +96,8 @@
 	// quickly silence build errors. This flag should be used with caution and only as a temporary
 	// measure, as it masks real errors and affects performance.
 	RelaxUsesLibraryCheck bool
+
+	EnableUffdGc bool // preopt with the assumption that userfaultfd GC will be used on device.
 }
 
 var allPlatformSystemServerJarsKey = android.NewOnceKey("allPlatformSystemServerJars")