package/gcc: switch to gcc 6.x as the default

package/gcc: switch to gcc 6.x as the default

Even if gcc 6 is still maintained for some time (gcc 6.5 is pending),
switch to gcc 7.x since it has been released since 2017-05-02 and
gcc 8.x is available since 2018-05-02.

We have been having toolchains in the autobuilders with gcc
7.x for a while, so the vast majority of the problems should have
already been solved.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>