arm: Add an export parameter to the const macro

This is currently not used in dav1d (yet), but there's a need for
it in rav1e, which shares this header with dav1d.
33 jobs for arm-export-const in 3 minutes and 39 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Style
passed #400833
amd64 docker
style-check

00:00:16

 
  Build
passed #400844
amd64 docker
build-android-aarch64

00:00:19

passed #400843
amd64 docker
build-android-armv7

00:00:19

passed #400834
amd64 avx2 docker
build-debian

00:00:28

passed #400845
aarch64 docker
build-debian-aarch64

00:01:10

passed #400846
aarch64 docker
build-debian-aarch64-clang-5

00:00:53

passed #400849
armv7 docker
build-debian-armv7

00:00:40

passed #400850
armv7 docker
build-debian-armv7-clang-5

00:00:33

passed #400837
amd64 docker
build-debian-examples

00:00:29

passed #400852
ppc64le docker
build-debian-ppc64le

00:00:50

passed #400835
amd64 docker
build-debian-static

00:00:31

passed #400848
aarch64 docker
build-debian-werror

00:00:30

passed #400836
amd64 docker
build-debian32

00:00:27

passed #400847
macos
build-macos

00:00:36

passed #400853
amd64 docker
build-pages

00:00:16

passed #400851
amd64 docker
build-ubuntu-snap

00:00:27

passed #400841
amd64 docker
build-win-arm32

00:00:20

passed #400842
amd64 docker
build-win-arm64

00:00:20

passed #400838
amd64 docker
build-win32

00:00:33

passed #400839
amd64 docker
build-win32-unaligned-stack

00:00:31

passed #400840
amd64 docker
build-win64

00:00:38

 
  Test
passed #400854
amd64 docker
test-debian

00:00:33

passed #400863
aarch64 docker
test-debian-aarch64

00:02:09

passed #400865
armv7 docker
test-debian-armv7-clang-5

00:00:49

passed #400859
amd64 docker
test-debian-asan

00:01:13

passed #400855
avx2 amd64 docker
test-debian-asm

00:00:52

passed #400860
amd64 docker
test-debian-msan

00:01:00

passed #400857
amd64 docker
test-debian-mt

00:00:37

passed #400864
ppc64le docker
test-debian-ppc64le

00:01:24

passed #400861
amd64 docker
test-debian-ubsan

00:00:59

passed #400858
amd64 avx2 docker
test-debian-unaligned-stack

00:00:48

passed #400856
avx2 amd64 docker
test-debian32-asm

00:01:11

passed #400862
amd64 avx2 docker
test-win64

00:00:50