Support cross compiling kernels with GCC6

* aportgen: modify code to allow generating gcc6-armhf and other gcc6
  cross compiler packages
* package: when 'gcc6' is in the depends of a package, and the cross
  compiling mode is "native" (as we do it with kernels), install the
  gcc6 cross compiler instead of the usual one (gcc8)

Related: pmaports#103