re-enable SSE2 code on x64 except with gcc

stb_image.h

@@ -673,7 +673,7 @@ typedef unsigned char validate_uint32[sizeof(stbi__uint32)==4 ? 1 : -1];
 #define STBI_NO_SIMD
 #endif
 
-#if !defined(STBI_NO_SIMD) && defined(STBI__X86_TARGET)
+#if !defined(STBI_NO_SIMD) && (defined(STBI__X86_TARGET) || defined(STBI__X64_TARGET))
 #define STBI_SSE2
 #include <emmintrin.h>