node-test-commit-arm should use select-compiler.sh

[richardlau]

While testing nodejs/node#38807 I noticed that we're using the default gcc/g++ binaries for the new arm64 containers.
e.g. https://ci.nodejs.org/job/node-test-commit-arm/38847/nodes=ubuntu1804-arm64/consoleFull

17:10:53 python3 ./configure --verbose 
17:10:56 Node.js configure: Found Python 3.6.9...
17:10:56 Detected C++ compiler (CXX=ccache g++) version: 7.5.0
17:10:56 WARNING: C++ compiler (CXX=ccache g++, 7.5.0) too old, need g++ 8.3.0 or clang++ 8.0.0
17:10:56 Detected C compiler (CC=ccache gcc) version: 7.5.0

We are installing the compilers into the containers, e.g.

build/ansible/roles/docker/templates/ubuntu1804.Dockerfile.j2

Lines 18 to 21 in a655f1a

	g++-6 \
	gcc-6 \
	g++-8 \
	gcc-8 \

but we're not using them. The job config currently hardcodes compiler selection: https://ci.nodejs.org/job/node-test-commit-arm/configure

  if [[ "$nodes" =~ centos[67]-(arm)?64-gcc8 ]]; then
    exec_cmd=". /opt/rh/devtoolset-8/enable; $exec_cmd"
  fi
  
  if [[ "$nodes" =~ centos[67]-(arm)?64-gcc6 ]]; then
    exec_cmd=". /opt/rh/devtoolset-6/enable; $exec_cmd"
  fi

i.e. currently only selects compilers for CentOS. I suggest we should use the select-compiler.sh script like we do elsewhere and extend for the Ubuntu containers. There are already arm64 entries for CentOS:

build/jenkins/scripts/select-compiler.sh

Lines 186 to 201 in a655f1a

	elif [ "$SELECT_ARCH" = "ARM64" ]; then
	echo "Setting compiler for Node version $NODEJS_MAJOR_VERSION on arm64"
	case $nodes in
	centos7-arm64-gcc8 )
	. /opt/rh/devtoolset-8/enable
	echo "Compiler set to devtoolset-8"
	;;
	centos[67]-arm64-gcc6 )
	. /opt/rh/devtoolset-6/enable
	echo "Compiler set to devtoolset-6"
	;;
	esac
	fi

[richardlau]

I've changed https://ci.nodejs.org/job/node-test-commit-arm/ to use select-compiler.sh (audit trail: https://github.com/nodejs/jenkins-config-test/commit/36ae5de59baee59e23731f399f39777285f1b913).
Opened #2753 to add compiler selection for Ubuntu 18.04.