contrib: vpx: disable neon_dotprod with Apple Clang 12

It fails to compile with the provided intrinsics although the code is correct.

../vpx_dsp/arm/vpx_convolve8_neon_dotprod.c:55:19: error: initializing 'int32x4_t' (vector of 4 'int32_t' values) with an expression of incompatible type '__attribute__((__vector_size__(4 * sizeof(uint32_t)))) uint32_t' (vector of 4 'uint32_t' values)
  int32x4_t sum = vdotq_lane_s32(acc, perm_samples, filters, 0);
                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/12.0.0/include/arm_neon.h:37021:11: note: expanded from macro 'vdotq_lane_s32'
int32x4_t __reint1 = __builtin_shufflevector(*(uint32x2_t *) &__reint, *(uint32x2_t *) &__reint, __p3, __p3, __p3, __p3); \
          ^          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fixes #29292 (closed)