These files are used to bisecting RX 540 gpu kernel bug, also can be used or kernels older than 4.16 or so: casper-rw.xz in this repository has all packages necessary to build kernels, you will still need to configure them on your host machine.
ubuntu 16.04.6 iso file with md5sum of 5416371cc0e990871746ddaac89f1a5e, you can download it from: http://releases.ubuntu.com/16.04/ubuntu-16.04.6-desktop-amd64.iso
yumi: i have used yumi 2.0.6.0
First extract casper-rw.xz, a 4gb casper-rw file will be created (this is persistent storage that includes all necessary packages, but its only compatible with ubuntu 16.04.6)
Then install iso file using yumi in a windows environment (i have used virtualbox for that), create 4gb persistent storage and replace extracted casper-rw file with that
Boot into usb and build kernels without any problem :) !
For benchmarking, i am using Xonotic and The Talos Principle
P.S: please see actual issue page: https://bugzilla.kernel.org/show_bug.cgi?id=201077