How to enable GCC 8.1 for tests?

Hello there! GCC 8.1 released May 2, 2018
So, I have it installed under my Gentoo and want to try compile live image with new compiler.
But in latest git snapshot new GCC not yet available.

My questions:

  1. how to add in "make menuconfig" this version of GCC?
  2. I want also compile live image with own toolchain. Please, post link or some notes how to configure?

PS I have some evidence for testing embedded Linux for satellite receivers and want know more how to stuff works. Thank You!