Gcc need to be downgraded from 6 to 5, because Gcc 6 is too new for
the latest Ubuntu LTS (16.04). This means Devtoolset-4 need to be used
instead of Devtoolset-6.

Task-number: QTBUG-64820
Change-Id: I2601aaaff5f3bbbadd867105fd4155239ab89825
Reviewed-by: Konstantin Tokarev <annulen@yandex.ru>
(cherry picked from commit adcdf7b4

)
Reviewed-by: Tony Sarajärvi <tony.sarajarvi@qt.io>