Don't use the Sk4f gradient impl without SIMD

src/opts/SkNx_neon.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkNx_neon_DEFINED
 #define SkNx_neon_DEFINED
 
+#define SKNX_IS_FAST
+
 namespace {  // See SkNx.h
 
 // Well, this is absurd.  The shifts require compile-time constant arguments.
 
 #define SHIFT8(op, v, bits) switch(bits) { \
     case  1: return op(v,  1);  case  2: return op(v,  2);  case  3: return op(v,  3); \
     case  4: return op(v,  4);  case  5: return op(v,  5);  case  6: return op(v,  6); \
     case  7: return op(v,  7); \
     } return fVec
 
@@ skipping 299 matching lines @@
 #undef SHIFT8
 
 template<>
 inline SkNx<4, int> SkNx_cast<int, float, 4>(const SkNx<4, float>& src) {
     return vcvtq_s32_f32(src.fVec);
 }
 
 }  // namespace
 
 #endif//SkNx_neon_DEFINED