toolchain: gcc: support -mmxu

This patch allows enabling support for MXU instructions on the GCC
command line with -mmxu.
This does not change anything in GCC code generation at the moment but
the flag is passed through to binutils, so that the instructions can be
used inline.
Merge pull request #9 from laanwj/gcc-mxu

toolchain: gcc: support -mmxu