loongarch: Rename SIMD helper intrinsics to avoid macro path issues

The `is::`, `cs::`, and `ls::` prefixes were originally used to clearly
distinguish helper intrinsics from different namespaces at call sites.

However, due to limitations in Rust macro expansion, `prefix::simd_xxx`
cannot always be used. Some macro expansions only accept an identifier,
causing errors such as:

```
  ($ft:literal, $name:ident, $op:path, $oty:ty, $ity:ident) => {
      let r: $ity = $op::<IMM, _>(a);
                       ^^ expected one of `.`, `;`, `?`, `else`, or an operator
```

Instead of relying on namespace prefixes, rename the LoongArch-specific
`simd_xxx` helpers to `simd_ext_xxx` and remove the `is::`, `cs::`, and
`ls::` prefixes from macro invocations. This preserves the distinction
between helper intrinsics while avoiding the macro expansion limitation.

Author: heiher