Compile with -x assembler-with-cpp instead of -assembler-with-cpp.

- Newer avr-gcc doesn't use -assembler-with-cpp, but
  uses -x assembler-with-cpp. This works with older compilers as well.

Author: Jimmy Hedman

platform.txt

@@ -15,7 +15,7 @@ compiler.c.cmd=arm-none-eabi-gcc
 compiler.c.flags=-c -g -Os -w -ffunction-sections -fdata-sections -nostdlib --param max-inline-insns-single=500 -Dprintf=iprintf
 compiler.c.elf.cmd=arm-none-eabi-g++
 compiler.c.elf.flags=-Os -Wl,--gc-sections
-compiler.S.flags=-c -g -assembler-with-cpp
+compiler.S.flags=-c -g -x assembler-with-cpp
 compiler.cpp.cmd=arm-none-eabi-g++
 compiler.cpp.flags=-c -g -Os -w -ffunction-sections -fdata-sections -nostdlib --param max-inline-insns-single=500 -fno-rtti -fno-exceptions -Dprintf=iprintf
 compiler.ar.cmd=arm-none-eabi-ar