Tracking issue for release notes of #150732: Convert `-Ctarget-cpu` into a target-modifier for AVR, AMDGCN and NVPTX

This issue tracks the release notes text for #150732.

cc @kulst, @bjorn3 -- original issue/PR authors and assignees for drafting text

See the forge.rust-lang.org chapter about release notes for an overview of how the release team makes use of these tracking issues.

Release notes text

This section should be edited to specify the correct category(s) for the change, with succinct description(s) of what changed. Some things worth considering:

• Does this need an additional compat notes section?
• Was this a libs stabilization that should have additional headers to list new APIs under Stabilized APIs and Const Stabilized APIs?

# Compiler
- [Convert `-Ctarget-cpu` into a target-modifier for AVR, AMDGCN and NVPTX](https://github.com/rust-lang/rust/pull/150732)

[!TIP]
Use the previous releases for inspiration on how to write the release notes text and which categories to pick.

Release blog section

If this change is notable enough for inclusion in the blog post then this section should be edited to contain a draft for the blog post. Otherwise leave it empty.

[!NOTE]

If a blog post section is required the release-blog-post label should be added (@rustbot label +release-blog-post) to this issue as otherwise it may be missed by the release team.

Start with the release notes text in this issue, then read the Forge release-notes chapter and recent releases for category and wording guidance. Confirm the description accurately summarizes PR #150732, add compatibility notes or a blog draft only if needed, and leave the release-note section ready for the release team.