[cpp] Broken code generated from objc externs

[tobil4sk]

To keep track of a few objective c extern tests that were disabled in #11123.

In 76cac7b, member access for extern classes was changed from -> to .. This is fine for properties with getters, but for regular member variables without getters, this causes objective c compiler errors:

Error: ./src/TestObjc.mm:89:53: error: no getter method for read from property
HXLINE(  26)            ::utest::Assert_obj::equals(this->cls.otherThing,42,null(),::hx::SourceInfo(HX_("TestObjc.hx",60,3f,d0,21),26,HX_("TestObjc",1e,aa,8d,26),HX_("testCall",10,9e,ec,44)));
                                                    ~~~~~~~~~~^~~~~~~~~~
./src/TestObjc.mm:134:53: error: no getter method for read from property
HXLINE(  47)            ::utest::Assert_obj::equals(this->cls.otherThing,142,null(),::hx::SourceInfo(HX_("TestObjc.hx",60,3f,d0,21),47,HX_("TestObjc",1e,aa,8d,26),HX_("testVar",b5,4d,c8,bd)));
                                                    ~~~~~~~~~~^~~~~~~~~~
2 errors generated.