msvc: do handle builds on Windows/ARM64

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>
[jc: adjust config.mak.uname for c18400c6]
Signed-off-by: Junio C Hamano <gitster@pobox.com>