Exit on unrecognized darwin arch hint

Not sure if Apple are to introduce another CPU architecture to their device lineup - but they already did it 2 times. When building this on arm64 macOS, before adding the arm64 arch hint, Configure with use64bitall wouldn't exit, but a rather cryptic clang error would be thrown for an unknown arch name, simply because nothing was passed after the -arch flag.

hints/darwin.sh

@@ -433,6 +433,7 @@ EOM
 *** Don't recognize processor, can't specify 64 bit compilation.
 
 EOM
+     exit 1
     ;;
     esac
     for var in ccflags cppflags ld ldflags