[CI] Select instruction set to avoid clang problems.
Clang 3.8 does not seem to have the intrinsic `_mm512_castps128_ps512()`, used in `vectorf128.h`:1063 for `approx_recipr()` if avx512f is available, but neither avx512er nor avx512vl are available. All currently existing processors have either avx512er (phi) or avx512vl (server-skylake). So do specify the latter to work around this.
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