Revert "Disable Global ThinLTO by default"

This reverts commit 9c81c0cfd94042e1c6406467e33de3564d647ac6.

Reason for revert: The ThinLTO problem has been fixed in https://android-review.git.corp.google.com/c/platform/bionic/+/2725997. So we can Re-enable Global ThinLTO.

Change-Id: I25dde4910692fd71124b48f45911da5b7b92a089
diff --git a/cc/lto.go b/cc/lto.go
index 820c1f0..df9ca0a 100644
--- a/cc/lto.go
+++ b/cc/lto.go
@@ -162,7 +162,7 @@
 }
 
 func GlobalThinLTO(ctx android.BaseModuleContext) bool {
-	return ctx.Config().IsEnvTrue("GLOBAL_THINLTO")
+	return !ctx.Config().IsEnvFalse("GLOBAL_THINLTO")
 }
 
 // Propagate lto requirements down from binaries