[llvm] [llvm-objcopy] Support SREC output format (PR #75874)

[James Henderson via llvm-commits]

https://github.com/jh7370 approved this pull request.

Aside from one nit (unnecessary `virtual` I believe), I think this PR now gets an LGTM from me.

A couple of caveats:

- I haven't attempted to review this against the SREC docs: I don't have time to do this, so I'm assuming that @statham-arm has done so.
- I haven't attempted to verify that the testing covers everything. If you have access to a code coverage tool, it might be instructive to run it through your modified llvm-objcopy with the tests you've written, to see if you've covered all the new code you've added.

https://github.com/llvm/llvm-project/pull/75874