Add support for new architectures

Ensure that builds and tests are sent to machines with the right architecture, so that something like skylake_avx512-centos7-gcc9-opt uses a build node labeled as skylake_avx512-build and a test node labeled as skylake_avx512-tests.

For the system to work, nodes have to be labeled with one <arch>-(build|tests) for each supported architectures. For example, for skylake_avx512 builds the labels have to be skylake_avx512-build, skylake-build, broadwell-build, haswell-build, nehalem-build, x86_64-build (although one may decide to omit some of them to reserve the machine for the specific architecture).