LoongArch64 LSX fast-path for str.contains(&str)

Benchmark results with LLVM 21 on LA664:

```
OLD:
test bench_is_contained_in ... bench:          43.63 ns/iter (+/- 0.04)

NEW:
test bench_is_contained_in ... bench:          12.81 ns/iter (+/- 0.01)
```

Author: heiher

library/core/src/str/pattern.rs

@@ -996,7 +996,10 @@ impl<'b> Pattern for &'b str {
                     return haystack.as_bytes().contains(&self.as_bytes()[0]);
                 }
 
-                #[cfg(all(target_arch = "x86_64", target_feature = "sse2"))]
+                #[cfg(any(
+                    all(target_arch = "x86_64", target_feature = "sse2"),
+                    all(target_arch = "loongarch64", target_feature = "lsx")
+                ))]
                 if self.len() <= 32 {
                     if let Some(result) = simd_contains(self, haystack) {
                         return result;
@@ -1770,11 +1773,18 @@ impl TwoWayStrategy for RejectAndMatch {
 /// If we ever ship std with for x86-64-v3 or adapt this for other platforms then wider vectors
 /// should be evaluated.
 ///
+/// Similarly, on LoongArch the 128-bit LSX vector extension is the baseline,
+/// so we also use `u8x16` there. Wider vector widths may be considered
+/// for future LoongArch extensions (e.g., LASX).
+///
 /// For haystacks smaller than vector-size + needle length it falls back to
 /// a naive O(n*m) search so this implementation should not be called on larger needles.
 ///
 /// [0]: http://0x80.pl/articles/simd-strfind.html#sse-avx2
-#[cfg(all(target_arch = "x86_64", target_feature = "sse2"))]
+#[cfg(any(
+    all(target_arch = "x86_64", target_feature = "sse2"),
+    all(target_arch = "loongarch64", target_feature = "lsx")
+))]
 #[inline]
 fn simd_contains(needle: &str, haystack: &str) -> Option<bool> {
     let needle = needle.as_bytes();
@@ -1906,7 +1916,10 @@ fn simd_contains(needle: &str, haystack: &str) -> Option<bool> {
 /// # Safety
 ///
 /// Both slices must have the same length.
-#[cfg(all(target_arch = "x86_64", target_feature = "sse2"))] // only called on x86
+#[cfg(any(
+    all(target_arch = "x86_64", target_feature = "sse2"),
+    all(target_arch = "loongarch64", target_feature = "lsx")
+))]
 #[inline]
 unsafe fn small_slice_eq(x: &[u8], y: &[u8]) -> bool {
     debug_assert_eq!(x.len(), y.len());