Resolve deprecation warning for 32 byte vectors

[JinShil]

This will remove the deprecation warning

src/core/simd.d(53): Deprecation: 32 byte vector types are only supported with -mcpu=avx

currently polluting druntime build output and facilitate advancement of the deprecation to an error.

[dlang-bot]

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Testing this PR locally

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[wilzbach]

Awesome! We should soon follow with fixing this to use -de like we do for Phobos, so that we don't end up in this state for years again.

[JinShil]

We should soon follow with fixing this to use -de

I would if I knew immediately where to put it.

[wilzbach]

#1982 (i.e. adding -de to DFLAGS)