arm32: mc: NEON implementation of warp8x8 for 16 bpc

Checkasm benchmarks:
                    Cortex A7      A8     A53     A72     A73
warp_8x8_16bpc_neon:   4062.6  2109.4  2462.0  1338.9  1391.1
warp_8x8t_16bpc_neon:  3996.3  2102.4  2412.0  1273.8  1368.9

Corresponding numbers for arm64, for comparison:
                                   Cortex A53     A72     A73
warp_8x8_16bpc_neon:                   2037.0  1148.8  1222.0
warp_8x8t_16bpc_neon:                  2008.0  1120.4  1200.9
33 jobs for master in 4 minutes and 55 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Style
passed #489649
amd64 docker
style-check

00:00:12

passed #489650
amd64 docker
x86inc-check

00:00:12

 
  Build
passed #489661
amd64 docker
build-android-aarch64

00:00:10

passed #489660
amd64 docker
build-android-armv7

00:00:20

passed #489651
amd64 avx2 docker
build-debian

00:00:34

passed #489662
aarch64 docker
build-debian-aarch64

00:00:25

passed #489663
aarch64 docker
build-debian-aarch64-clang-5

00:00:25

passed #489666
armv7 docker
build-debian-armv7

00:00:43

passed #489667
armv7 docker
build-debian-armv7-clang-5

00:00:36

passed #489654
amd64 docker
build-debian-examples

00:00:30

passed #489669
ppc64le docker
build-debian-ppc64le

00:01:26

passed #489652
amd64 docker
build-debian-static

00:00:34

passed #489665
aarch64 docker
build-debian-werror

00:00:21

passed #489653
amd64 docker
build-debian32

00:00:23

passed #489664
macos
build-macos

00:00:58

passed #489668
amd64 docker
build-ubuntu-snap

00:00:30

passed #489658
amd64 docker
build-win-arm32

00:00:10

passed #489659
amd64 docker
build-win-arm64

00:00:12

passed #489655
amd64 docker
build-win32

00:00:30

passed #489656
amd64 docker
build-win32-unaligned-stack

00:00:26

passed #489657
amd64 docker
build-win64

00:00:42

 
  Test
passed #489670
amd64 docker
test-debian

00:00:59

97.9711%
passed #489679
aarch64 docker
test-debian-aarch64

00:00:37

passed #489681
armv7 docker
test-debian-armv7-clang-5

00:00:49

passed #489675
amd64 docker
test-debian-asan

00:01:47

passed #489671
avx2 amd64 docker
test-debian-asm

00:00:50

passed #489676
amd64 docker
test-debian-msan

00:00:40

passed #489673
amd64 docker
test-debian-mt

00:00:31

passed #489680
ppc64le docker
test-debian-ppc64le

00:02:12

passed #489677
amd64 docker
test-debian-ubsan

00:00:57

passed #489674
amd64 avx2 docker
test-debian-unaligned-stack

00:00:31

passed #489672
avx2 amd64 docker
test-debian32-asm

00:01:06

passed #489678
amd64 avx2 docker
test-win64

00:00:56