Add Narwhal GNU site config, add self-hosted CI runner on macOS

[climbfuji]

Description

1. Add a new CI test running on a github self-hosted runner managed by JCSDA (.github/workflows/macos-ci-x86_64.yaml).
2. Add Narwhal GNU site config, tested successfully with jedi-bundle on the machine by @climbfuji and Caroline P. (Univ. of San Diego).
3. Fix inconsistent documentation for macOS w.r.t. Python, and update Linux documentation to match how we build Python nowadays.

• Waiting on Bug fix for mapl when environment variable BASEDIR is set spack#229

Testing

Note. The existing CI tests have definitely reached the end of their life, irreparably broken.

The following tests passed:

• CI macOS see https://github.com/NOAA-EMC/spack-stack/actions/runs/4305070571/jobs/7506988940
• macos-dom see https://github.com/NOAA-EMC/spack-stack/actions/runs/4304702923/jobs/7506097964 (timed out just before the end, but everything else built!)
• Narwhal+GNU