Generate ABI dumps for libraries without source

This commit enables ABI diff for libraries without source.
Stub libraries are excluded.

Test: ./create_reference_dumps.py -products aosp_arm64
Test: make
Bug: 135728451
Change-Id: Idbe276db6f5e057175fc098f0226cb38996faa17
diff --git a/cc/cc.go b/cc/cc.go
index 53ec899..a513a07 100644
--- a/cc/cc.go
+++ b/cc/cc.go
@@ -778,6 +778,10 @@
 		// APEX variants do not need ABI dumps.
 		return false
 	}
+	if ctx.isStubs() {
+		// Stubs do not need ABI dumps.
+		return false
+	}
 	if ctx.isNdk() {
 		return true
 	}