Add GCC 7 (#4144)

* Add GCC 7
* Allow users to build subset of GCC compilers with multi-value variant.
* Add comment explaining what 'all' actually means
* More specific golang support
* Only require Zip when building Java
* Comment out provide directives that don't currently work
* Add Ada support
* Use conflicts directive
* Fix joining of languages
* Need special flag to build jit
* Explicitly declare GNAT download extension
* Import tty, update lib64 to lib
* BRIG and Go are not supported on macOS
* Simplify formatting and imports
* JIT patch required for newer versions as well