World - Change default compiler from gcc-5 to gcc-8

World - Change default compiler from gcc-5 to gcc-8

* Change the default compiler from gcc-5 to gcc-8.
  gcc-5 will no longer be built by default.  Setting
  WORLD_ALTCOMPILER=all will build all three as before.

* Create CC hardlink for gcc80 so dports picks it up.
  gcc80 is now the default for dports instead of 50.

* Adjust the Makefile's to make things easier the next time,
  though they could still use more work.