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GH-40602: [C++] Move mold linker flags to variables (#40603)

### 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: #40602

Lead-authored-by: Marcus D. Hanwell <marcus@voltrondata.com>
Co-authored-by: Marcus D. Hanwell <mhanwell@gmail.com>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>