Similarly to what we've just done to prevent both --with-arch and
--with-cpu, we do the same to prevent using both --with-cpu and
--with-tune at the same time, since --with-cpu should fully imply
the CPU to tune for (and gcc now errors out when both are specified.
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Bryan Hundven <bryanhundven@gmail.com>