enable some undocumented g++ optimizations for less code size

We don't need RTTI and exception handling, we only want our templates to be
evaluated at compile time. With this patch, code size is about the same as with
plain C defines.

Thanks to @Droggelbecher, who pointed me at the Wiselib Makefile:
https://github.com/ibr-alg/wiselib/blob/master/apps/iapps/wiselib_example/Makefile.msp430#L31