ui-macos/bits/runpython.do: auto-determine arches to build for.

Some people don't have all of them installed, so auto-detect them by
looking at the available arches in /usr/libexec.

ui-macos/bits/runpython.do

 exec >&2
 redo-ifchange runpython.c
-gcc -arch ppc -arch i386 -arch x86_64 \
+ARCHES=""
+for d in /usr/libexec/gcc/darwin/*; do
+    ARCHES="$ARCHES -arch $(basename $d)"
+done
+gcc $ARCHES \
 	-Wall -o $3 runpython.c \
 	-I/usr/include/python2.5 \
 	-lpython2.5