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GH-33786: [C++] Ignore old system xsimd (#33811)

### Rationale for this change

If old xsimd is installed, CMake target for bundled xsimd is conflicted.

### What changes are included in this PR?

Use `arrow::xsimd` for bundled xsimd's target name to avoid conflict.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* Closes: #33786

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>