arm32: filmgrain: Add NEON implementation of gen_grain for 8 bpc

Relative speedup over C code:

                             Cortex A7     A8     A9    A53    A72    A73
gen_grain_uv_ar0_8bpc_420_neon:   6.13   7.81   8.17   6.78   6.62  11.13
gen_grain_uv_ar0_8bpc_422_neon:   6.34   7.64   8.00   6.83   6.93  10.31
gen_grain_uv_ar0_8bpc_444_neon:   7.09   8.29   8.55   7.95   7.89  11.05
gen_grain_uv_ar1_8bpc_420_neon:   3.39   2.26   3.06   4.13   3.41   4.95
gen_grain_uv_ar1_8bpc_422_neon:   3.40   2.23   3.02   4.18   3.36   4.73
gen_grain_uv_ar1_8bpc_444_neon:   3.46   2.18   2.95   4.46   3.57   4.91
gen_grain_uv_ar2_8bpc_420_neon:   3.88   3.00   3.32   4.74   3.57   5.31
gen_grain_uv_ar2_8bpc_422_neon:   3.92   3.04   3.36   4.82   3.57   5.06
gen_grain_uv_ar2_8bpc_444_neon:   4.32   3.14   3.62   5.56   3.90   5.43
gen_grain_uv_ar3_8bpc_420_neon:   4.35   3.53   4.05   5.35   4.44   5.56
gen_grain_uv_ar3_8bpc_422_neon:   4.38   3.49   4.17   5.41   4.48   5.36
gen_grain_uv_ar3_8bpc_444_neon:   4.84   3.70   4.36   5.95   4.87   5.82
gen_grain_y_ar0_8bpc_neon:        5.18   5.57   7.65   5.93   7.13   9.01
gen_grain_y_ar1_8bpc_neon:        2.64   1.66   2.48   3.32   3.15   3.77
gen_grain_y_ar2_8bpc_neon:        3.57   2.64   3.21   4.59   3.68   4.64
gen_grain_y_ar3_8bpc_neon:        4.27   3.93   4.12   5.41   4.63   5.17

(A73 is benched against C code compiled with a different C compiler,
which can explain the slightly differing numbers there.)

Absolute numbers:

                                 Cortex A7         A8         A9        A53       A72        A73
gen_grain_uv_ar0_8bpc_420_neon:    19614.6    13396.4    12320.4    15030.7    8288.1     8754.4
gen_grain_uv_ar0_8bpc_422_neon:    34660.9    24315.5    22225.3    26809.2   14549.8    15804.6
gen_grain_uv_ar0_8bpc_444_neon:    55625.6    39914.5    37100.2    44658.3   22917.3    27369.6
gen_grain_uv_ar1_8bpc_420_neon:    50049.5    63179.4    44793.1    36406.7   22690.3    25401.9
gen_grain_uv_ar1_8bpc_422_neon:    93289.5   117755.0    82815.4    67081.4   43133.1    46698.0
gen_grain_uv_ar1_8bpc_444_neon:   170880.0   223259.2   156241.5   122760.0   78655.6    85604.9
gen_grain_uv_ar2_8bpc_420_neon:    68185.5    78123.2    61457.3    47886.7   31526.2    36519.6
gen_grain_uv_ar2_8bpc_422_neon:   129195.2   148653.9   114133.2    89822.7   60242.6    70160.1
gen_grain_uv_ar2_8bpc_444_neon:   233133.7   272277.4   214108.7   161589.5  109069.3   127763.7
gen_grain_uv_ar3_8bpc_420_neon:    96374.4    94372.2    79663.8    70832.0   43065.3    50593.9
gen_grain_uv_ar3_8bpc_422_neon:   186324.8   184321.8   151490.1   136200.1   83758.0    98378.7
gen_grain_uv_ar3_8bpc_444_neon:   335596.6   336811.6   279755.5   247251.5  151657.2   178906.0
gen_grain_y_ar0_8bpc_neon:         46109.3    36022.2    28476.2    36478.5   18740.1    20660.4
gen_grain_y_ar1_8bpc_neon:        165054.2   217090.4   152578.9   118409.4   74357.2    83794.5
gen_grain_y_ar2_8bpc_neon:        226576.9   268320.3   210924.6   157829.4  105956.5   124293.2
gen_grain_y_ar3_8bpc_neon:        328337.2   330421.3   275110.1   242097.3  148538.7   177270.8

Corresponding numbers for the original arm64 version:

                                                                 Cortex A53       A72        A73
gen_grain_uv_ar0_8bpc_420_neon:                                     14874.7    7765.5     8536.0
gen_grain_uv_ar0_8bpc_422_neon:                                     26510.9   13685.3    15308.2
gen_grain_uv_ar0_8bpc_444_neon:                                     43189.6   21565.3    24312.0
gen_grain_uv_ar1_8bpc_420_neon:                                     33715.7   21669.8    22758.3
gen_grain_uv_ar1_8bpc_422_neon:                                     63955.3   41581.4    42852.5
gen_grain_uv_ar1_8bpc_444_neon:                                    117390.1   76503.5    78446.4
gen_grain_uv_ar2_8bpc_420_neon:                                     42779.0   27794.3    29677.9
gen_grain_uv_ar2_8bpc_422_neon:                                     82283.8   53446.7    58232.2
gen_grain_uv_ar2_8bpc_444_neon:                                    147773.8   98492.7   103754.1
gen_grain_uv_ar3_8bpc_420_neon:                                     56698.8   35697.1    40695.9
gen_grain_uv_ar3_8bpc_422_neon:                                    110132.4   69829.1    79196.8
gen_grain_uv_ar3_8bpc_444_neon:                                    196642.7  124174.9   141812.5
gen_grain_y_ar0_8bpc_neon:                                          36461.0   17782.0    19827.0
gen_grain_y_ar1_8bpc_neon:                                         113202.7   72457.7    75995.8
gen_grain_y_ar2_8bpc_neon:                                         142894.0   94450.9   100304.5
gen_grain_y_ar3_8bpc_neon:                                         191697.7  120674.9   137223.8