Update the stdsimd submodule

This brings in a few updates:

* Update wasm intrinsic naming for atomics
* Update and reimplement most simd128 wasm intrinsics
* Other misc improvements here and there, including a small start to
  AVX-512 intrinsics

src/libcore/lib.rs

@@ -118,6 +118,7 @@
 #![feature(mips_target_feature)]
 #![feature(aarch64_target_feature)]
 #![feature(wasm_target_feature)]
+#![feature(avx512_target_feature)]
 #![feature(const_slice_len)]
 #![feature(const_str_as_bytes)]
 #![feature(const_str_len)]