GH-40602: [C++] Move mold linker flags to variables

[cryos]

Rationale for this change

Move the mold linker flags to the CMake CMAKE_*_LINKER_FLAGS variables. This is the more correct place to add them as they will only be used during linking, it would invalidate any [s]ccache less frequently if changing linkers and I think it is more clear/correct.

What changes are included in this PR?

Moving the mold linker flags from the CMAKE_*_FLAGS to CMAKE_*_LINKER_FLAGS.

Are these changes tested?

Existing tests and validation would cover these changes and the result is essentially the same as before.

Are there any user-facing changes?

No

• GitHub Issue: [C++] Add mold linker flags to the CMake linker flags variables #40602

[github-actions]

⚠️ GitHub issue #40602 has been automatically assigned in GitHub to PR creator.

[kou]

+1

Thanks. I couldn't remember these flags when I wrote this feature.

[cryos]

Thanks, I need to use string(APPEND ...) more, I had forgotten that had been added and love removing repetition!

[kou]

Merged.

BTW, do you want to work on related issue #40400 ?

[cryos]

Merged.

BTW, do you want to work on related issue #40400 ?

I could help with that, thanks @kou