UBUNTU: [Packaging]: always use fully qualified <triplet>-gcc-VER toolchain

UBUNTU: [Packaging]: always use fully qualified <triplet>-gcc-VER toolchain

Always define $(gcc) variable and export it. Always use fully
qualified <triplet>-gcc-VER toolchains for native (GNU build) and
cross (GNU host) compilation. Generate matching build-dependencies for
native and cross builds.

This ensures that native and cross builds use the same fully qualified
toolchain binary, fixed in time, irrespective of gcc-defaults updates.

Signed-off-by: Dimitri John Ledkov <dimitri.ledkov@canonical.com>
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>