Toolchain XML improvements

This is a pass to try and include any changes between the hxlibc and hxcpp toolchain XML, although I have not been able to fully test each target, this includes obvious differences I saw.

One of the changes in hxlibc was to allow for toolchain XML that is outside the HXCPP directory, using the include path. The previous pull request (I believe) makes a minor change to help allow for this.

Since an XML may not necessarily come from the build-tool directory, I've leveraged your new "ifExists" attribute to help it check the dependency only for core targets. It would be more ideal for this to be a little smarter, but for now, I believe it should resolve errors when using a toolchain that does not exist in HXCPP.

For reference, you would use a custom toolchain like this:

```` bash
haxelib run hxcpp Build.xml -Dtoolchain=newplatform -Ipath/to/other/dir
````

Similar to the core targets, it would search for "newplatform-toolchain.xml" under all the include directories, including the new one added. If I get a chance to test, I'll submit any minor tweaks if it still doesn't quite work as expected.

This resolves a typo in the iOS toolchain, and it makes some updates for Emscripten