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)

(reviews would be nice, but this is so urgent that I'll continue ASAP anyway even if nobody reviews it. I know that this will probably not pass CI right now.)

Related: pmaports#103 (closed)