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

firmware/export/config.h

@@ -1075,7 +1075,7 @@ Lyre prototype 1 */
  * Older versions of GCC emit assembly in divided syntax with no option
  * to enable unified syntax.
  */
-#if (__GNUC__ < 8)
+#if (__GNUC__ < 8) && defined(CPU_ARM_CLASSIC)
 #define BEGIN_ARM_ASM_SYNTAX_UNIFIED ".syntax unified\n"
 #define END_ARM_ASM_SYNTAX_UNIFIED   ".syntax divided\n"
 #else

lib/rbcodec/codecs/libfaad/sbr_qmf.c

@@ -460,7 +460,8 @@ void sbr_qmf_synthesis_64(sbr_info *sbr, qmfs_info *qmfs, qmf_t X[MAX_NTSR][64],
         p_buf_1 = qmfs->v + qmfs->v_index;
 
         /* calculate 64 output samples and window */
-#ifdef CPU_ARM
+        /* note: GCC 4.9 complains about the inline asm on Cortex-M targets */
+#if defined(CPU_ARM_CLASSIC) || (defined(CPU_ARM_MICRO) && __GNUC__ > 4)
         const real_t *qtab = qmf_c;
         real_t *pbuf = p_buf_1;
         for (k = 0; k < 64; k++, pbuf++)

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