allow aarch64_softfloat_neon for backwards compatibility

[RalfJung]

This should help unblock rust-lang/rust#135160.

[rustbot]

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[RalfJung]

Looks like the repo needs an update before CI works again (or it should use a pinned nightly, that would avoid such problems):

error: the feature `avx512_target_feature` has been stable since 1.89.0-nightly and no longer requires an attribute to enable
  --> crates/core_arch/src/lib.rs:25:5
   |
25 |     avx512_target_feature,
   |     ^^^^^^^^^^^^^^^^^^^^^

[folkertdev]

CI has been fixed now