Warning when using target-feature prefer-256-bit

When using the target features prefer-128-bit or prefer-256-bit, the compiler throws the warning:

warning: unknown feature specified for `-Ctarget-feature`: `prefer-256-bit`
  |
  = note: it is still passed through to the codegen backend
  = note: consider filing a feature request

This is even though the features are listed in the rustc --print=target-features list, and actually works. From this code, disabling prefer-256-bit clearly works as avx-512 registers are used in the generated assembly.

The warning said to consider filing a feature request, so here I am. Hope this can be fixed.