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.
Merge pull request #1814 from HeMan/master

Update for newer avr-gcc. (solves https://github.com/arduino/Arduino/issues/1807)