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>

util/dockerfiles/ubuntu-20.04_gcc-version/Dockerfile

@@ -0,0 +1,50 @@
+# Copyright (c) 2020 The Regents of the University of California
+# All Rights Reserved.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met: redistributions of source code must retain the above copyright
+# notice, this list of conditions and the following disclaimer;
+# redistributions in binary form must reproduce the above copyright
+# notice, this list of conditions and the following disclaimer in the
+# documentation and/or other materials provided with the distribution;
+# neither the name of the copyright holders nor the names of its
+# contributors may be used to endorse or promote products derived from
+# this software without specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+# A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+# OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+FROM ubuntu:20.04
+
+# Valid version values:
+# 7
+# 8
+# 9
+# 10
+ARG version
+
+ENV DEBIAN_FRONTEND=noninteractive
+RUN apt -y update
+RUN apt -y upgrade
+RUN apt -y install git m4 scons zlib1g zlib1g-dev libprotobuf-dev \
+    protobuf-compiler libprotoc-dev libgoogle-perftools-dev python3-dev \
+    python3-six python-is-python3 doxygen libboost-all-dev libhdf5-serial-dev \
+    python3-pydot libpng-dev gcc-${version} g++-${version}
+
+RUN update-alternatives --install \
+    /usr/bin/g++ g++ /usr/bin/g++-${version} 100
+RUN update-alternatives --install \
+    /usr/bin/gcc gcc /usr/bin/gcc-${version} 100
+RUN update-alternatives --install \
+    /usr/bin/c++ c++ /usr/bin/g++-${version} 100
+RUN update-alternatives --install \
+    /usr/bin/cc cc /usr/bin/gcc-${version} 100