Allow cc_genrule srcs to vary by architecture

Add arch_variant to the genrule srcs property so that cc_genrule
modules can vary the srcs and outs by architecture.

Test: cc/genrule_test.go
Change-Id: Idb17013e137d7ca21ca4cfc3e4c1fa7d89977043
diff --git a/Android.bp b/Android.bp
index bcf831d..5faefe6 100644
--- a/Android.bp
+++ b/Android.bp
@@ -173,6 +173,7 @@
     testSrcs: [
         "cc/cc_test.go",
         "cc/gen_test.go",
+        "cc/genrule_test.go",
         "cc/library_test.go",
         "cc/test_data_test.go",
         "cc/util_test.go",