bugfix: external detection for compilers with os but not target (#44156)

avoid calling `spec.target` when None.

When an external compiler package has an `os` set but no `target` set, Spack
currently falls into a codepath that calls `spec.target` (which itself calls
`spec.architecture.target.Microarchitecture`) when `spec.architecture.target`
is None, throwing an error.

e.g.

```
packages:
  gcc:
    externals:
    - spec: gcc@12.3.1 os=rhel7
      prefix: /usr
```

---------

Co-authored-by: Todd Gamblin <tgamblin@llnl.gov>