Deprecate and Remove [target_machine] from cross file format

[Ericson2314]

Meson does nothing itself with the target machine. I think its a bit misleading that it parses it at all when only user-written logic can use it. I've talked in #3972 about having separate host and target files rather than just one cross file. I think it would be best to:

1. Remove [target_machine]
2. Rename [host_machine] to [machine]
3. Add [machine] to native platform for consistency.
4. Use a separate target file if we ever wish to support target platforms "for real"

[Ericson2314]

Note how there is PerMachine and PerThreeMachine in mesonlib. With this change, the last use of PerThreeMachine will be become PerMachine so we can just delete that class. Much better!