mdx: inherit link_flags from env stanza for mdx_gen executable

[Copilot]

Description

The generated mdx_gen.bc.exe was linked with only the MDX-specific -linkall argument, so link_flags from env stanzas were skipped. This made environment-level linker configuration ineffective for MDX, including flags used to suppress duplicate-library warnings on macOS.

This restores the existing expectation that env link flags apply when Dune links an executable.

Implementation

• Add a cram regression that configures -cclib -lm in a workspace env stanza and prints those flags from the mdx_gen.bc.exe linker trace.
• Evaluate MDX link arguments with Ocaml_flags_db.link_flags sctx ~dir Dune_lang.Link_flags.Spec.standard, the same default link-flag path used by ordinary executable rules.
• Keep the MDX-required -linkall argument prepended to the evaluated flags.
• Add a changelog entry.

Using Link_flags.Spec.standard avoids a separate env-only lookup and gives MDX the normal executable link-flag semantics. Since this restores existing behavior, it is applied as a bug fix rather than being gated on Dune language 3.25.

Test plan

• ./dune.exe runtest test/blackbox-tests/test-cases/stanzas/mdx-stanza/env-link-flags.t
• ./dune.exe fmt

Fixes #15613

Checklist

• Tests added.
• Change log entry added.
• Documentation added; not needed because this restores documented env behavior.

[Alizter]

@copilot address comments

[Alizter]

MDX executable link flags are now inherited from the relevant env stanzas. This initial PR was written by copilot and then iterated upon by me.

@avsm Can you confirm that this is what we want for the mdx executable? Addressing the ask in #15613.