Add stdarch-gen-common: shared check/bless harness for generators

[xonx4l]

This PR adds a library crate stdarch-gen-common that gives every generator three modes -:

1. Write - Generator writes files straight into the committed location.
2. Check - Generator write into a temporary directory . The crate diffs it against committed and fails on any mismatch without touching the working copy .
3. Bless - Generator writes into a temp directory the crate then mirrors temp over committed. Adding, overwriting, and deleting files as needed.

Mode is picked via the STDARCH_GEN_MODE env var. The harness only touches files the generator explicitly lists as its own so hand-written files in the same directory are safe.

This PR also hooks stdarch-gen-hexagon to use it as the first example.

r? @folkertdev

[Kobzol]

For a bit more context, this is done for the stdarch GSoC project, where we try to integrate stdarch's tests in rust-lang/rust's CI. We want to move the blessing logic out of the CI YAML files and into Rust, to make it easier to reuse, and also share code amongst the various generators.

I'll take a look, but also CC @folkertdev , in case you have in mind who owns this code amongst the stdarch maintainers and who could also chime in.