Hexagon assembler uses `.string` directive instead of `.asciz` for
null-terminated strings. Both are equivalent but the test was only
checking for `.asciz`.

Update the CHECK patterns to accept both directives using
`.{{asciz|string}}` regex pattern.

…modifiers

To make it clearer what happens. In other words, do not silently keep
modifiers as part of `AuxCrate::name`.

…bilee

add `simd_splat` intrinsic

Add `simd_splat` which lowers to the LLVM canonical splat sequence.

```llvm
insertelement <N x elem> poison, elem %x, i32 0
shufflevector <N x elem> v0, <N x elem> poison, <N x i32> zeroinitializer
```

Right now we try to fake it using one of

```rust
fn splat(x: u32) -> u32x8 {
    u32x8::from_array([x; 8])
}
```

or (in `stdarch`)

```rust
fn splat(value: $elem_type) -> $name {
    #[derive(Copy, Clone)]
    #[repr(simd)]
    struct JustOne([$elem_type; 1]);
    let one = JustOne([value]);
    // SAFETY: 0 is always in-bounds because we're shuffling
    // a simd type with exactly one element.
    unsafe { simd_shuffle!(one, one, [0; $len]) }
}
```

Both of these can confuse the LLVM optimizer, producing sub-par code. Some examples:

- rust-lang#60637
- rust-lang#137407
- rust-lang#122623
- rust-lang#97804

---

As far as I can tell there is no way to provide a fallback implementation for this intrinsic, because there is no `const` way of evaluating the number of elements (there might be issues beyond that, too). So, I added implementations for all 4 backends.

Both GCC and const-eval appear to have some issues with simd vectors containing pointers. I have a workaround for GCC, but haven't yet been able to make const-eval work. See the comments below.

Currently this just adds the intrinsic, it does not actually use it anywhere yet.

compiletest: Make `aux-crate` directive explicitly handle `--extern` modifiers

With `-Zunstable-options` it is possible to pass options to `--extern`. See here for an exhaustive list of possible options:
https://github.com/rust-lang/rust/blob/b5dd72d2921500c9d9e15f074e1d831adcaa3dee/compiler/rustc_session/src/config.rs#L2356-L2367

Using these options works with the `aux-crate` directive, but only because the options pretend to be part of the name. Make it clearer what `aux-crate` supports by explicitly handling `--extern` options.

This PR is step one of splitting up rust-lang#151258 into smaller pieces.

r? @Zalathar

…ss35

Fix cstring-merging test for Hexagon target

Hexagon assembler uses `.string` directive instead of `.asciz` for null-terminated strings. Both are equivalent but the test was only checking for `.asciz`.

Update the CHECK patterns to accept both directives using `.{{asciz|string}}` regex pattern.