Couple of fixes to allow SIMD on Emscripten

- Add `-msimd128` to flags to actually enable WebAssembly SIMD
   when performing SIMD detection. It's currently required in
   addition to `-msse*` / `-mfpu=neon` flags which only perform
   translation of corresponding intrinsics to Wasm SIMD ones.
   See a discussion at emscripten-core/emscripten#12714 for
   automating this and making easier in the future.
 - Remove compilation branch that prevented definitions of
   `WEBP_USE_SSE` and `WEBP_USE_NEON` on Emscripten even when
   SIMD support was detected at compile-time.
 - Add an implementation of `VP8GetCPUInfo` for Emscripten which
   uses static `WEBP_USE_*` flags to determine if a corresponding
   SIMD instruction is supported. This is because Wasm doesn't
   have proper feature detection (yet) and requires making separate
   build for SIMD version anyway.

Change-Id: I77592081b91fd0e4cbc9242f5600ce905184f506

src/dsp/dsp.h

@@ -51,9 +51,6 @@ extern "C" {
 # define __has_builtin(x) 0
 #endif
 
-// for now, none of the optimizations below are available in emscripten
-#if !defined(EMSCRIPTEN)
-
 #if defined(_MSC_VER) && _MSC_VER > 1310 && \
     (defined(_M_X64) || defined(_M_IX86))
 #define WEBP_MSC_SSE2  // Visual C++ SSE2 targets
@@ -110,8 +107,6 @@ extern "C" {
 #define WEBP_USE_MSA
 #endif
 
-#endif  /* EMSCRIPTEN */
-
 #ifndef WEBP_DSP_OMIT_C_CODE
 #define WEBP_DSP_OMIT_C_CODE 1
 #endif