arm64: filmgrain: Add NEON implementation of the generate_grain_y function

Relative speedup over C code:
                       Cortex A53    A72    A73   Apple M1
gen_grain_y_ar0_8bpc_neon:   5.03   5.18   5.46   5.22
gen_grain_y_ar1_8bpc_neon:   3.40   3.16   3.48   2.78
gen_grain_y_ar2_8bpc_neon:   4.99   4.11   5.02   3.99
gen_grain_y_ar3_8bpc_neon:   6.73   5.69   6.66   5.34
38 jobs for arm64-gen-grain-y in 7 minutes and 27 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Style
passed #615799
amd64 docker
style-check

00:00:15

passed #615800
amd64 docker
x86inc-check

00:00:12

 
  Build
passed #615815
amd64 docker
build-android-aarch64

00:00:10

passed #615814
amd64 docker
build-android-armv7

00:00:10

passed #615801
amd64 avx2 docker
build-debian

00:00:36

passed #615816
aarch64 docker
build-debian-aarch64

00:00:16

passed #615817
aarch64 docker
build-debian-aarch64-clang-5

00:00:14

passed #615820
armv7 docker
build-debian-armv7

00:01:54

passed #615821
armv7 docker
build-debian-armv7-clang-5

00:01:26

passed #615808
amd64 docker
build-debian-bitdepth-16

00:00:12

passed #615807
amd64 docker
build-debian-bitdepth-8

00:00:16

passed #615805
amd64 docker
build-debian-examples

00:00:17

passed #615803
amd64 avx2 docker
build-debian-illegal-instructions

00:01:46

passed #615806
amd64 docker
build-debian-no-tools

00:00:16

passed #615823
ppc64le docker
build-debian-ppc64le

00:01:16

passed #615802
amd64 docker
build-debian-static

00:00:25

passed #615819
aarch64 docker
build-debian-werror

00:00:09

passed #615804
amd64 docker
build-debian32

00:00:23

passed #615818
catalina amd64
build-macos

00:00:34

passed #615824
amd64 docker
build-pages

00:00:08

passed #615822
amd64 docker
build-ubuntu-snap

00:00:23

passed #615812
amd64 docker
build-win-arm32

00:00:15

passed #615813
amd64 docker
build-win-arm64

00:00:14

passed #615809
amd64 docker
build-win32

00:00:31

passed #615810
amd64 docker
build-win32-unaligned-stack

00:00:29

passed #615811
amd64 docker
build-win64

00:00:44

 
  Test
passed #615825
amd64 docker
test-debian

00:01:15

95.8317%
passed #615834
aarch64 docker
test-debian-aarch64

00:00:26

passed #615836
armv7 docker
test-debian-armv7-clang-5

00:03:43

passed #615829
amd64 docker
test-debian-asan

00:02:26

passed #615826
avx2 amd64 docker
test-debian-asm

00:00:52

passed #615830
amd64 docker
test-debian-msan

00:01:01

passed #615835
ppc64le docker
test-debian-ppc64le

00:02:14

passed #615832
amd64 docker
test-debian-tsan

00:02:44

passed #615831
amd64 docker
test-debian-ubsan

00:01:32

passed #615828
amd64 avx2 docker
test-debian-unaligned-stack

00:00:50

passed #615827
avx2 amd64 docker
test-debian32-asm

00:00:57

passed #615833
amd64 avx2 docker
test-win64

00:04:23