configure: fix PIC activation for x86_64 shared libs

Originally committed as revision 16563 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -2085,7 +2085,7 @@ fi
 if enabled shared; then
     # LIBOBJFLAGS may have already been set in the OS configuration
     if test -z "$LIBOBJFLAGS" ; then
-        case "$arch" in
+        case "${subarch-$arch}" in
             x86_64|ia64|alpha|sparc*|power*|parisc*|mips*) LIBOBJFLAGS='$(PIC)' ;;
         esac
     fi