mingw(arm64): do move the `/etc/git*` location

mingw(arm64): do move the `/etc/git*` location

In fb5e3378f8 (mingw: move Git for Windows' system config where users
expect it, 2021-06-22), I moved the location of Git for Windows' system
config and system Git attributes file to the top-level `/etc/` directory
(because it is a much more obvious location than, say, `/mingw64/etc/`).

The patch relied on a very specific scenario that the newly-supported
Windows/ARM64 builds of `git.exe` fails to fall into. So let's broaden
the condition a bit, so that Windows/ARM64 builds also use that location
(instead of the even more obscure `/clangarm64/etc/` directory).

This fixes https://github.com/git-for-windows/git/issues/5431.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>