Arbitrary implementations for floats trigger compiler warnings

[carolynzech]

Upgrading our toolchain to 2/7/25, then running ./scripts/kani-regression.sh now produces this warning:

   Compiling kani_core v0.59.0 (/Users/cmzech/kani/library/kani_core)
warning: target feature `neon` must be enabled to ensure that the ABI of the current target can be implemented correctly                                                                                                                                                                
  |
  = note: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #116344 <https://github.com/rust-lang/rust/issues/116344>


warning: target feature `neon` must be enabled to ensure that the ABI of the current target can be implemented correctly                                                                                                                                                                
  |
  = note: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #116344 <https://github.com/rust-lang/rust/issues/116344>


   Compiling std v0.59.0 (/Users/cmzech/kani/library/std)
warning: target feature `neon` must be enabled to ensure that the ABI of the current target can be implemented correctly                                                                                                                                                                
  |
  = note: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #116344 <https://github.com/rust-lang/rust/issues/116344>

My hunch is that our trivial_arbitrary! implementations for floats are triggering this warning. The solution may just be to enable this target feature, but we need to dig more into the linked issue to ensure that's the right course of action.

[tautschnig]

I think we should just enable that target_feature to silence the warning: Kani does not care about the ABI for it uses its own interpretation of MIR and never deals with the actual object code generated for the target platform.