arm: workaround to build Cortex-M7 targets with GCC 4.9

Cortex-M7 support was added in GCC 5, while GCC 4.9 only
supports the M4. The instruction set is almost identical
between both processors; the only difference is that the
M7 supports double-precision floating point and the M4
doesn't.

Since Rockbox currently doesn't use the FPU, building M7
targets as M4 works fine.

Change-Id: I5880d6e81a85fa9b3e16e08d57e7955b4493df0b

tools/configure

@@ -4617,6 +4617,16 @@ else
    GCCOPTS="$GCCOPTS -Wno-expansion-to-defined -Wimplicit-fallthrough=0"
  fi
 
+ case "$GCCOPTS" in
+   *-mcpu=cortex-m7*)
+     # Cortex-M7 support wasn't added until GCC 5. Since we don't use any
+     # M7-specific features, we can compile as Cortex-M4 when on GCC 4.9.
+     if test "$gccnum" -lt "500"; then
+       GCCOPTS=$(echo "$GCCOPTS" | sed -e "s/-mcpu=cortex-m7/-mcpu=cortex-m4 -mthumb/")
+     fi
+     ;;
+ esac
+
  case $prefix in
    ""|"$CROSS_COMPILE")
      # simulator