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

avr/platform.txt

@@ -17,7 +17,7 @@ compiler.c.cmd=avr-gcc
 compiler.c.flags=-c -g -Os -w -ffunction-sections -fdata-sections -MMD
 compiler.c.elf.flags=-Os -Wl,--gc-sections
 compiler.c.elf.cmd=avr-gcc
-compiler.S.flags=-c -g -assembler-with-cpp
+compiler.S.flags=-c -g -x assembler-with-cpp
 compiler.cpp.cmd=avr-g++
 compiler.cpp.flags=-c -g -Os -w -fno-exceptions -ffunction-sections -fdata-sections -MMD
 compiler.ar.cmd=avr-ar