Xcode toolchain compatibility

The generated Xcode toolchain is (now?) incompatible with Xcode 8 on
Sierra. A few tweaks are necessary for Xcode 8:

1- rename the plist to ToolchainInfo.plist
2- use the default Xcode toolchain as a more "modern" template
3- change compatibility id to 2 instead of 1

There may be some other necessary tweaks, but at least these keep
Xcode happy.

Just FYI for anyone struggling with this. I will update this bug
if necessary.