Fix darwin arch hint on arm64 when using use64bitall

When calling Configure with use64bitall on arm64 macOS, it compiles a test program using clang and an -arch flag that is not followed by anything. Seems like the darwin arch hint when using use64bitall couldn't work with the 'arm' result from uname -p.

Author: mtsanovv

hints/darwin.sh

@@ -427,6 +427,7 @@ EOM
     case `uname -p` in 
     powerpc) arch=ppc64 ;;
     i386) arch=x86_64 ;;
+    arm) arch=arm64 ;;
     *) cat <<EOM >&4
 
 *** Don't recognize processor, can't specify 64 bit compilation.