help: special-case HOST_CPU universal

When building Git as a universal binary on macOS, the binary supports more than
one target architecture. This is a bit of a problem for the `HOST_CPU`
setting that is woefully unprepared for such a situation, as it wants to
show architecture hard-coded at build time.

In preparation for releasing universal builds, work around this by
special-casing `universal` and replacing it at run-time with the known
values `x86_64` or `arm64`.

Signed-off-by: Jeff Hostetler <jeffhostetler@github.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

help.c

@@ -730,6 +730,22 @@ const char *help_unknown_cmd(const char *cmd)
 	exit(1);
 }
 
+#if defined(__APPLE__)
+static const char *git_host_cpu(void) {
+	if (!strcmp(GIT_HOST_CPU, "universal")) {
+#if defined(__x86_64__)
+		return "x86_64";
+#elif defined(__aarch64__)
+		return "arm64";
+#endif
+	}
+
+	return GIT_HOST_CPU;
+}
+#undef GIT_HOST_CPU
+#define GIT_HOST_CPU git_host_cpu()
+#endif
+
 void get_version_info(struct strbuf *buf, int show_build_options)
 {
 	/*