Some clangarm64 fixes (#5439)

Most notably, this fixes
#5431, i.e. that the ARM64
flavor of Git for Windows does not use `/etc/gitconfig` by mistake, but
`/clangarm64/etc/gitconfig` instead.

My original intention was to fix only that issue, but when I fired up a
Git for Windows/ARM64 SDK and tried to build, I immediately ran into
trouble, then noticed places where ARM64 was not yet handled, and one
thing led to another and now it's a 6-patch PR instead of a single-patch
one.

Author: dscho

config.mak.uname

@@ -432,7 +432,11 @@ ifeq ($(uname_S),Windows)
         ifeq (MINGW32,$(MSYSTEM))
 		prefix = /mingw32
         else
-		prefix = /mingw64
+		ifeq (CLANGARM64,$(MSYSTEM))
+			prefix = /clangarm64
+		else
+			prefix = /mingw64
+		endif
         endif
 	# Prepend MSVC 64-bit tool-chain to PATH.
 	#
@@ -486,7 +490,7 @@ ifeq ($(uname_S),Windows)
 	NATIVE_CRLF = YesPlease
 	DEFAULT_HELP_FORMAT = html
 	SKIP_DASHED_BUILT_INS = YabbaDabbaDoo
-ifeq (/mingw64,$(subst 32,64,$(prefix)))
+ifeq (/mingw64,$(subst 32,64,$(subst clangarm,mingw,$(prefix))))
 	# Move system config into top-level /etc/
 	ETC_GITCONFIG = ../etc/gitconfig
 	ETC_GITATTRIBUTES = ../etc/gitattributes
@@ -748,7 +752,7 @@ ifeq ($(uname_S),MINGW)
 	USE_LIBPCRE = YesPlease
 	USE_MIMALLOC = YesPlease
 	NO_PYTHON =
-        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