vpx_convolve_copy_neon: fix unaligned loads w/w==4

vpx_convolve_copy_neon: fix unaligned loads w/w==4

Fixes a -fsanitize=undefined warning:

vpx_dsp/arm/vpx_convolve_copy_neon.c:29:26: runtime error: load of
misaligned address 0xffffa8242bea for type 'const uint32_t' (aka 'const
unsigned int'), which requires 4 byte alignment
0xffffa8242bea: note: pointer points here
 88 81  7d 7d 7d 7d 7d 81 81 7d  81 80 87 97 a8 ab a0 91 ...
              ^
    #0 0xb0447c in vpx_convolve_copy_neon
       vpx_dsp/arm/vpx_convolve_copy_neon.c:29:26
    #1 0x12285c8 in inter_predictor vp9/common/vp9_reconinter.h:29:3
    #2 0x1228430 in dec_build_inter_predictors
       vp9/decoder/vp9_decodeframe.c
    ...

Change-Id: Iaec4ac2a400b6e6db72d12e5a7acb316262b12a7