ARM: enable thumb for Cortex-M* CPUs

These CPUs are thumb-only and thus require this option.
Signed-off-by: NMans Rullgard <mans@mansr.com>

configure

@@ -2270,7 +2270,7 @@ elif enabled arm; then
             case $cpu in
                 cortex-a*)                               subarch=armv7a  ;;
                 cortex-r*)                               subarch=armv7r  ;;
-                cortex-m*)                               subarch=armv7m  ;;
+                cortex-m*)                 enable thumb; subarch=armv7m  ;;
                 arm11*)                                  subarch=armv6   ;;
                 arm[79]*e*|arm9[24]6*|arm96*|arm102[26]) subarch=armv5te ;;
                 armv4*|arm7*|arm9[24]*)                  subarch=armv4   ;;