Update the target list to Rust 1.48.0

[sunshowers]

Checklist

• I have read the Contributor Guide
• I have read and agree to the Code of Conduct
• I have added a description of my changes and why I'd like them included in the section below

Description of Changes

Update the builtin target list to Rust 1.48.

(Could you release this as cfg-expr 0.5.1? Thanks!)

[sunshowers]

Ahh, it looks like bytecodealliance/target-lexicon#63 needs to be addressed first.

[Jake-Shadle]

I've added a workaround for the issue you linked, I think anyone actually using that target in any capacity would be rare at this point, thanks for the PR!

[sunshowers]

Thank you!