msvc: do handle builds on Windows/ARM64

Git for Windows/ARM64 settled on using `clang` to compile `git.exe`, and
hence needs to run in a system where `MSYSTEM` is set to `CLANGARM64`
and the prefix to use is `/clangarm64`.

We already did that in the `MINGW` arm, i.e. for regular Git for Windows
builds using MINGW GCC (or `clang`'s shim pretending to be GCC), now it
is time to do the same in the MS Visual C part.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

config.mak.uname

@@ -439,7 +439,11 @@ 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.
 	#