System with multiple compilers

[pearce8]

Description

When some of the dependencies don't build with the default compiler, we use this syntax to provide an alternative compiler::

packages:
  all:
    require:
    - one_of: ["%cce", "%gcc"]  # this line makes spack use cce to build where possible, gcc otherwise

• Depends on Refactor Systems to Initialize Using Dictionaries #658
• What is this called in Spack, and can we link to where this is explained?
• How to detect that Spack is building a secondary compiler?
• This should be explained in how to add a system
• Add a test to verify that only the special packages use the secondary compiler to be added to CI
• altdiff option to show which compiler was used to build which dependence