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>

compat/mingw.h

@@ -6,6 +6,25 @@ typedef _sigset_t sigset_t;
 #include <winsock2.h>
 #include <ws2tcpip.h>
 
+#ifdef __MINGW64_VERSION_MAJOR
+/*
+ * In Git for Windows, we cannot rely on `uname -m` to report the correct
+ * architecture: /usr/bin/uname.exe will report the architecture with which the
+ * current MSYS2 runtime was built, not the architecture for which we are
+ * currently compiling (both 32-bit and 64-bit `git.exe` is built in the 64-bit
+ * Git for Windows SDK).
+ */
+#undef GIT_HOST_CPU
+/* This was figured out by looking at `cpp -dM </dev/null`'s output */
+#if defined(__x86_64__)
+#define GIT_HOST_CPU "x86_64"
+#elif defined(__i686__)
+#define GIT_HOST_CPU "i686"
+#else
+#error "Unknown architecture"
+#endif
+#endif
+
 /* MinGW-w64 reports to have flockfile, but it does not actually have it. */
 #ifdef __MINGW64_VERSION_MAJOR
 #undef _POSIX_THREAD_SAFE_FUNCTIONS