package/gcc: switch to gcc 10.x as the default

package/gcc: switch to gcc 10.x as the default

Even if gcc 9.x is still maintained for some time (gcc 9.5 will be the
last), switch to gcc 10.x since it has been released since 2020-05-07
and gcc 11.x is available since 2021-04-27.

We have been having toolchains in the autobuilders with gcc 10.x since
mid-January 2021, so the vast majority of the problems should have
already been solved.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>