Fix CPU detection for armhf

This applies the same fix in MythTV as was applied in 75cf504 to FFmpeg.
The same code is in MythTV and needs the same fix.

Fixes #505

(cherry picked from commit 13e75f1)

mythtv/configure

@@ -4308,9 +4308,10 @@ elif enabled arm; then
         fi
     }
 
-    [ "$cpu" = generic ] && cpu=$(probe_arm_arch)
-
     case $cpu in
+        generic)
+            subarch=$(probe_arm_arch | sed 's/[^a-z0-9]//g')
+        ;;
         armv*)
             cpuflags="-march=$cpu"
             subarch=$(echo $cpu | sed 's/[^a-z0-9]//g')