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

In fb5e337 (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 git-for-windows#5431.

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

Author: dscho

config.mak.uname

@@ -489,7 +489,7 @@ ifeq ($(uname_S),Windows)
 	NO_POSIX_GOODIES = UnfortunatelyYes
 	NATIVE_CRLF = YesPlease
 	DEFAULT_HELP_FORMAT = html
-ifeq (/mingw64,$(subst 32,64,$(prefix)))
+ifeq (/mingw64,$(subst 32,64,$(subst clangarm,mingw,$(prefix))))
 	# Move system config into top-level /etc/
 	ETC_GITCONFIG = ../etc/gitconfig
 	ETC_GITATTRIBUTES = ../etc/gitattributes
@@ -749,7 +749,7 @@ ifeq ($(uname_S),MINGW)
         ifneq (CLANGARM64,$(MSYSTEM))
 		USE_NED_ALLOCATOR = YesPlease
         endif
-        ifeq (/mingw64,$(subst 32,64,$(prefix)))
+        ifeq (/mingw64,$(subst 32,64,$(subst clangarm,mingw,$(prefix))))
 		# Move system config into top-level /etc/
 		ETC_GITCONFIG = ../etc/gitconfig
 		ETC_GITATTRIBUTES = ../etc/gitattributes