arm64: mc: Reduce the width of a register copy

Only copy as much as really is needed/used.
26 jobs for master in 6 minutes and 36 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Style
passed style-check #353897
amd64 debian

00:00:21

 
  Build
passed build-debian #353898
amd64 debian

00:00:42

passed build-debian-aarch64 #353906
debian aarch64

00:01:13

passed build-debian-aarch64-clang-5 #353907
debian aarch64

00:00:52

passed build-debian-armv7 #353910
debian armv7

00:02:07

passed build-debian-armv7-clang-5 #353911
debian armv7

00:01:28

passed build-debian-ppc64le #353913
docker ppc64le

00:00:52

passed build-debian-static #353899
amd64 debian

00:00:41

passed build-debian-werror #353909
debian aarch64

00:00:30

passed build-debian32 #353900
amd64 debian

00:00:40

passed build-macos #353908
macos

00:00:27

passed build-ubuntu-snap #353912
amd64 debian

00:01:03

passed build-win-arm32 #353904
amd64 debian

00:00:26

passed build-win-arm64 #353905
amd64 debian

00:00:29

passed build-win32 #353901
amd64 debian

00:00:47

passed build-win32-unaligned-stack #353902
amd64 debian

00:00:43

passed build-win64 #353903
amd64 debian

00:00:50

 
  Test
passed test-debian #353914
amd64 debian

00:00:50

passed test-debian-aarch64 #353920
debian aarch64

00:02:12

passed test-debian-armv7-clang-5 #353922
debian armv7

00:02:32

passed test-debian-asan #353916
amd64 debian

00:02:33

passed test-debian-msan #353917
amd64 debian

00:01:23

passed test-debian-ppc64le #353921
docker ppc64le

00:01:26

passed test-debian-ubsan #353918
amd64 debian

00:01:38

passed test-debian-unaligned-stack #353915
amd64 debian

00:00:48

passed test-win64 #353919
amd64 debian

00:00:56