TEST: Move to gcc-11.0 prerelease and LLVM-11

- gcc-11.0 proper is coming soon to kernel.org, use this snapshot for
  now to see what fails.
- grub-2.04 does not build with gcc-11.0 due to -Werror, grub-2.06-rc1
  fails on riscv32-linux as seen here:
  https://www.mail-archive.com/grub-devel@gnu.org/msg30736.html



Signed-off-by: Tom Rini <trini@konsulko.com>