util: Added Dockerfile for GCC versions on ubuntu 20.04

util: Added Dockerfile for GCC versions on ubuntu 20.04

Ubuntu 20.04 contains GCC verisons 9 and 10, which are not easily
obtainable (at least through APT) on Ubuntu 18.04. Therefore, a
Dockerfile for obtaining GCC versions in Ubuntu 20.04 has been added.
The orignal GCC version Dockerfile (Ubuntu 18.04) has been kept as
GCC versions 4.8, 5, and 6 are not obtainable, via APT, on Ubuntu
20.04. A complete migration to the 20.04 Dockerfile is not possible
until these earlier GCC versions are dropped.

The Docker images for GCC Versions 9 and 10 can be found here:
https://gcr.io/gem5-test/gcc-version-10
https://gcr.io/gem5-test/gcc-version-9

The other Dockerfile directories have been renamed for consistency.

Change-Id: I569249331095ee62d1be5be479c7ba7da0077422
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/30514
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>