ld: generate checksums on release and open pull request

[crazy-max]

Adds a new step in ld workflow to create a GitHub Release, generate ld checksums and open a pull request.

Tested on my fork:

• Run https://github.com/crazy-max/xx/actions/runs/5825405077/job/15797759809
• Release https://github.com/crazy-max/xx/releases/tag/prebuilt%2Fld-2.38-11
• PR https://github.com/crazy-max/xx/pull/5/commits/c72006dde99cc914eef21163272c34efd16472a3

[crazy-max]

In https://github.com/crazy-max/xx/pull/5/commits/c72006dde99cc914eef21163272c34efd16472a3 I see 386 and arm/v6 archs are missing for prebuilt ld, I will look at this.

Edit: opened #120 to fix missing platforms