mingw: fix CPU reporting in `git version --build-options`

mingw: fix CPU reporting in `git version --build-options`

We cannot rely on `uname -m` in Git for Windows' SDK to tell us what
architecture we are compiling for, as we can compile both 32-bit and
64-bit `git.exe` from a 64-bit SDK, but the `uname -m` in that SDK will
always report `x86_64`.

So let's go back to our original design. And make it explicitly
Windows-specific.

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