Don't pass any debug options directly to the assembler

Don't pass any debug options directly to the assembler

When passing the -g option to gcc, gcc automatically selects a
suitable --g<format> option to pass on to the assembler.
Thus, there's no point in forcing a specific debug option on the
assembler using the -Wa mechanism.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>