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GH-36663: [C++] Fix the default value information for enum options (#36684)

### Rationale for this change

The default for `ARROW_SIMD_LEVEL` is described as `NONE` but it's actually `DEFAULT`.
The default for `ARROW_RUNTIME_SIMD_LEVEL` is described as `NONE` but it's actually `MAX`.

### What changes are included in this PR?

Reorder possible values to put the default value as the first element.

Before:

    --   ARROW_SIMD_LEVEL=SSE4_2 [default=NONE|SSE4_2|AVX2|AVX512|NEON|SVE|SVE128|SVE256|SVE512|DEFAULT]
    --   ARROW_RUNTIME_SIMD_LEVEL=MAX [default=NONE|SSE4_2|AVX2|AVX512|MAX]

After:

    --   ARROW_SIMD_LEVEL=SSE4_2 [default=DEFAULT|NONE|SSE4_2|AVX2|AVX512|NEON|SVE|SVE128|SVE256|SVE512]
    --   ARROW_RUNTIME_SIMD_LEVEL=MAX [default=MAX|NONE|SSE4_2|AVX2|AVX512]

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.
* Closes: #36663

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>