concretizer: handle compiler existence check settings
concretizer: handle compiler existence check settings

To handle unknown compilers propely in tests (and elsewhere), we need to
add unknown compilers from the spec to the list of possible compilers.

Rework how the compiler list is generated and includes compilers from
specs if the existence check is disabled.

LLNL/spack,iulian787/spack,iulian787/spack,LLNL/spack,iulian787/spack,iulian787/spack,LLNL/spack,LLNL/spack,iulian787/spack,LLNL/spack