Refactor U8 CLAMP microkernels

- Reimplement SSE2 and NEON versions
- Add WAsm SIMD implementation
- Refactor unit tests and remove the no longer used VClampMicrokernelTester

PiperOrigin-RevId: 391169964
diff --git a/BUILD.bazel b/BUILD.bazel
index bd79371..4aebcea 100644
--- a/BUILD.bazel
+++ b/BUILD.bazel
@@ -1834,6 +1834,7 @@
     "src/qu8-vmulc/gen/minmax-fp32-wasmsimd-mul32-ld64-x16.c",
     "src/s8-maxpool/9p8x-minmax-wasmsimd-c16.c",
     "src/u8-maxpool/9p8x-minmax-wasmsimd-c16.c",
+    "src/u8-vclamp/wasmsimd-x64.c",
     "src/x32-packx/x4-wasmsimd.c",
     "src/x32-unpool/wasmsimd.c",
     "src/x32-zip/x2-wasmsimd.c",