hardware: platform.txt tweak from upstream

Commit: fb324358eeff5c82ffba7ace495f26a26c1715b4 [fb32435]
Author: Jimmy Hedman <jimmy.hedman@southpole.se>
Date: 21 January 2014 20:57:35 GMT

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.