cmake/BuildArrow: Use AUTO mode for xsimd dependency detection

cmake/BuildArrow: Use AUTO mode for xsimd dependency detection

Arrow requires xsimd >= 9.0.1 according to arrow/cpp/thirdparty/versions.txt.
Previously, we unconditionally set -Dxsimd_SOURCE=BUNDLED, forcing the use
of Arrow's vendored xsimd regardless of system package availability.

This commit changes to -Dxsimd_SOURCE=AUTO, which allows Arrow's
resolve_dependency mechanism to automatically:
1. Try to find system xsimd package
2. Check if version >= 9.0.1
3. Use system version if found and sufficient
4. Fall back to bundled version otherwise

This reduces build time and dependencies on systems with sufficient xsimd,
while maintaining compatibility with older distributions.

Distribution availability:
- Ubuntu Noble (24.04): libxsimd-dev 12.1.1 (✓ will use system)
- Ubuntu Jammy (22.04): libxsimd-dev 7.6.0 (✗ will use bundled)
- Debian Trixie (13): libxsimd-dev 13.2.0 (✓ will use system)
- CentOS Stream 9: xsimd-devel 7.4.9 (✗ will use bundled)

Signed-off-by: Kefu Chai <k.chai@proxmox.com>