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 #1816 from HeMan/ide-1.5.x

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