* Assemble through `gcc` instead of `as` directly, so source files are
preprocessed (i.e., `#include`)
* `gcc` does not pass `-I` include directories to `as`. This can
supposedly be fixed by recompiling `gcc` with `--with-gnu-as`. Popular
N64 toolchain binary distributions don't do this, and to lower the
barrier to entry I don't want to force users to have to compile their
own. Work around it by passing `-Wa,-I`.
* CMake only finds assembly dependencies when preprocessing, not when
assembling. Use `#include` instead of `.include` to work around this.
The `.incbin`s used by `sound_data.s` are covered by the
sound_data_tables target dependency.
* Refactor entry point
* Remove unused code
* Reformat level header to be more clear, and add more comments
Looks like assembling with CMake will require some changes to the
assembler flags, and possibly some manual dependency specification.
