arm: Consistently use 8/24 columns indentation for assembly

For cases with indented, nested .if/.macro in asm.S, ident those
by 4 chars.

Some initial assembly files were indented to 4/16 columns, while all
the actual implementation files, starting with src/arm/64/mc.S, have
used 8/24 for indentation.
21 jobs for paeth in 8 minutes and 10 seconds (queued for 1 second)
Status Job ID Name Coverage
  Style
passed #268880
amd64 debian
style-check

00:00:25

 
  Build
passed #268881
amd64 debian
build-debian

00:00:39

passed #268889
debian aarch64
build-debian-aarch64

00:01:27

passed #268890
debian aarch64
build-debian-aarch64-clang-5

00:01:05

passed #268893
debian armv7
build-debian-armv7

00:01:26

passed #268894
debian armv7
build-debian-armv7-clang-5

00:01:11

passed #268882
amd64 debian
build-debian-static

00:00:37

passed #268892
debian aarch64
build-debian-werror

00:00:34

passed #268883
amd64 debian
build-debian32

00:00:37

passed #268891
macos
build-macos

00:00:32

passed #268895
amd64 debian allowed to fail
build-ubuntu-snap

00:00:44

passed #268887
amd64 debian
build-win-arm32

00:00:31

passed #268888
amd64 debian
build-win-arm64

00:00:32

passed #268884
amd64 debian
build-win32

00:00:40

passed #268885
amd64 debian
build-win32-unaligned-stack

00:00:39

passed #268886
amd64 debian
build-win64

00:00:47

 
  Test
passed #268896
amd64 debian
test-debian

00:00:51

passed #268897
amd64 debian
test-debian-asan

00:02:19

passed #268898
amd64 debian
test-debian-msan

00:01:12

passed #268899
amd64 debian
test-debian-ubsan

00:01:31

passed #268900
amd64 debian
test-win64

00:00:47