Explicitly set OBJC=cc OBJCXX=c++ for bitcode test

OBJC=clang / OBJCXX=clang++ can pick up a newer Clang that no longer
supports bitcode.

test cases/osx/7 bitcode/test.json

@@ -0,0 +1,6 @@
+{
+  "env": {
+    "OBJC": "cc",
+    "OBJCXX": "c++"
+  }
+}