packetizer/startcode_helper: disable AVX2 vector-size implementation

generates the following errors on some platforms (android-x86;
android-x86_64; macos; ios-simulator-x86_64):

../../modules/packetizer/startcode_helper.h:91:41: error: invalid input size for constraint 'x'
                : [v]"r"(p), [czero]"x"(zeros)

'x' is supposedly the right constraint for both xmm and ymm registers, and
this compiles fine on other platforms. googling got me nowhere.

modules/packetizer/startcode_helper.h

@@ -75,23 +75,24 @@ static inline const uint8_t * startcode_FindAnnexB_AVX2( const uint8_t *p, const
     alignedend = end - ((intptr_t) end & 31);
     if( alignedend > p )
     {
-#  ifdef HAS_ATTRIBUTE_VECTORSIZE
-        const v32qu zeros = { 0 };
-#  endif
+//#  ifdef HAS_ATTRIBUTE_VECTORSIZE
+//        const v32qu zeros = { 0 };
+//#  endif
 
         for( ; p < alignedend; p += 32)
         {
             uint32_t match;
-#  ifdef HAS_ATTRIBUTE_VECTORSIZE
-            __asm__ volatile(
-                "vmovdqa    0(%[v]),   %%ymm0\n"
-                "vpcmpeqb   %[czero],  %%ymm0, %%ymm0\n"
-                "vpmovmskb  %%ymm0,    %[match]\n" /* mask will be in reversed match order */
-                : [match]"=r"(match)
-                : [v]"r"(p), [czero]"x"(zeros)
-                : "ymm0"
-            );
-#  else
+//FIXME: This gives this error on some platforms: "invalid input size for constraint 'x'"
+//#  ifdef HAS_ATTRIBUTE_VECTORSIZE
+//            __asm__ volatile(
+//                "vmovdqa    0(%[v]),   %%ymm0\n"
+//                "vpcmpeqb   %[czero],  %%ymm0, %%ymm0\n"
+//                "vpmovmskb  %%ymm0,    %[match]\n" /* mask will be in reversed match order */
+//                : [match]"=r"(match)
+//                : [v]"r"(p), [czero]"x"(zeros)
+//                : "ymm0"
+//            );
+//#  else
             __asm__ volatile(
                 "vmovdqa    0(%[v]),  %%ymm0\n"
                 "vpxor      %%ymm1,   %%ymm1, %%ymm1\n"
@@ -101,7 +102,7 @@ static inline const uint8_t * startcode_FindAnnexB_AVX2( const uint8_t *p, const
                 : [v]"r"(p)
                 : "ymm0", "ymm1"
             );
-#  endif
+//#  endif
             if( match & 0x0000000F )
                 TRY_MATCH(p, 0);
             if( match & 0x000000F0 )