Provide an environment variable to disable mlgo

Helps iterate on compiler that was build without mlgo. Building with mlgo requires a docker image

Change-Id: Ie2988ecaf15031cdb14411abf7b99530e3860cce
diff --git a/cc/lto.go b/cc/lto.go
index fb3b485..ad1a1b1 100644
--- a/cc/lto.go
+++ b/cc/lto.go
@@ -147,10 +147,12 @@
 			}
 		}
 
-		// Register allocation MLGO flags for ARM64.
-		if ctx.Arch().ArchType == android.Arm64 {
-			ltoCFlags = append(ltoCFlags, "-mllvm -regalloc-enable-advisor=release")
-			ltoLdFlags = append(ltoLdFlags, "-Wl,-mllvm,-regalloc-enable-advisor=release")
+		if !ctx.Config().IsEnvFalse("THINLTO_USE_MLGO") {
+			// Register allocation MLGO flags for ARM64.
+			if ctx.Arch().ArchType == android.Arm64 {
+				ltoCFlags = append(ltoCFlags, "-mllvm -regalloc-enable-advisor=release")
+				ltoLdFlags = append(ltoLdFlags, "-Wl,-mllvm,-regalloc-enable-advisor=release")
+			}
 			// Flags for training MLGO model.
 			if ctx.Config().IsEnvTrue("THINLTO_EMIT_INDEXES_AND_IMPORTS") {
 				ltoLdFlags = append(ltoLdFlags, "-Wl,--save-temps=import")