makefile: add an helper target to genie xcode targets

[biappi]

This patch exposes the Xcode genie generator in the main makefile, similarly to the vs2022 phony target. It seems to work fine, modulo some caveats, that I also noted down as comments:

• the generator does not support the build tasks to autogenerate files

• there are issues with path handling that results in a slightly broken project file

• some build products will clobber the working copy.

  As reference, this is the status after a build.

  willy@Hitagi mame add-genie-xcode-generator-to-makefile$ git status
  On branch add-genie-xcode-generator-to-makefile
  Your branch is up to date with 'origin/add-genie-xcode-generator-to-makefile'.
  
  Untracked files:
    (use "git add <file>..." to include in what will be committed)
      scripts/mame_mame/
      scripts/src/lib7z.a
      scripts/src/libbgfx.a
      scripts/src/libbimg.a
      scripts/src/libbx.a
      scripts/src/libemu.a
      scripts/src/libexpat.a
      scripts/src/libflac.a
      scripts/src/libjpeg.a
      scripts/src/liblinenoise.a
      scripts/src/liblua.a
      scripts/src/liblualibs.a
      scripts/src/libportaudio.a
      scripts/src/libportmidi.a
      scripts/src/libsoftfloat.a
      scripts/src/libsoftfloat3.a
      scripts/src/libsqlite3.a
      scripts/src/libutf8proc.a
      scripts/src/libutils.a
      scripts/src/libwdlfft.a
      scripts/src/libymfm.a
      scripts/src/libzlib.a
      scripts/src/libzstd.a
      scripts/src/mame/libfrontend.a
      scripts/src/osd/libqtdbg_sdl.a
      scripts/src/osd/mame_mame/
  
  nothing added to commit but untracked files present (use "git add" to track)
  willy@Hitagi mame add-genie-xcode-generator-to-makefile$