WAsm SIMD version of X32 UNPOOL microkernel

- Port X32 UNPOOL microkernel to WAsm SIMD
- Slightly refactor other versions of X32 UNPOOL microkernel

PiperOrigin-RevId: 321694416
diff --git a/BUILD.bazel b/BUILD.bazel
index 57d7e77..28bf329 100644
--- a/BUILD.bazel
+++ b/BUILD.bazel
@@ -815,6 +815,7 @@
     "src/x32-fill/wasmsimd.c",
     "src/x32-packx/x4-wasmsimd.c",
     "src/x32-pad/wasmsimd.c",
+    "src/x32-unpool/wasmsimd.c",
     "src/math/roundne-wasmsimd-addsub.c",
     "src/math/roundd-wasmsimd-addsub.c",
     "src/math/roundd-wasmsimd-cvt.c",