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

cmake/cpu.cmake

@@ -70,6 +70,9 @@ foreach(I_SIMD RANGE ${WEBP_SIMD_FLAGS_RANGE})
   webp_check_compiler_flag(${WEBP_SIMD_FLAG} ${WEBP_ENABLE_SIMD})
   if(NOT WEBP_HAVE_${WEBP_SIMD_FLAG})
     list(GET SIMD_ENABLE_FLAGS ${I_SIMD} SIMD_COMPILE_FLAG)
+    if(EMSCRIPTEN)
+      set(SIMD_COMPILE_FLAG "-msimd128 ${SIMD_COMPILE_FLAG}")
+    endif()
     set(CMAKE_REQUIRED_FLAGS ${SIMD_COMPILE_FLAG})
     webp_check_compiler_flag(${WEBP_SIMD_FLAG} ${WEBP_ENABLE_SIMD})
   else()