Use #![feature(const_panic)] to produce better assertion errors

[Aaron1011]

Currently, core_arch uses 1 / (bool_expr as usize) to generate a
post-monomorphization error when a condition is false. However, this
approach generates opaque error messages
('evaluation of constant value failed'), which could easily be mistaken
for an internal compiler error by users.

Using the const_panic feature, we can panic!() with a more
descriptive error message. Unfortunately, we cannot include any of the
actual values in our message, since const traits do not yet exist.

Helps with rust-lang/rust#85155

[rust-highfive]

r? @Amanieu

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[Amanieu]

Have you checked that this actually produces a better error message in the cross-crate case?

[Aaron1011]

@Amanieu: Yes. I added the failing example from rust-lang/rust#85155 (comment) to examples/hex.rs

let a: __m128 = _mm_setzero_ps();
let b: __m128 = _mm_setzero_ps();
let _blended = _mm_blend_ps(a, b, 0x33);

and got the following error:

error[E0080]: evaluation of constant value failed
 --> /home/aaron/repos/stdarch/crates/core_arch/src/macros.rs:8:9
  |
8 |         assert!(IMM >= MIN && IMM <= MAX, "IMM value not in expected range");
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the evaluated program panicked at 'IMM value not in expected range', /home/aaron/repos/stdarch/crates/core_arch/src/macros.rs:8:9
  |
  = note: this error originates in the macro `$crate::panic::panic_2015` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to previous error

For more information about this error, try `rustc --explain E0080`.
error: could not compile `stdarch_examples`

There's still no backtrace, but that's something that will need to be addressed on the rustc side.