gcc: switch to gcc 5.x as the default

gcc: switch to gcc 5.x as the default

gcc 4.9.4 was the last release of the 4.9.x branch, and the gcc
developes will now only be maintaining gcc 5.x and 6.x:

  https://gcc.gnu.org/ml/gcc-announce/2016/msg00002.html

Therefore, it is time to use gcc 5.x as the default version in
Buildroot. We have been having toolchains in the autobuilders with gcc
5.x for a while, so the vast majority of the problems should have
already been solved.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>