build: Replace GCC ARM Embedded with GNU Arm Embedded

The old GCC ARM Embedded website on launchpad
(https://launchpad.net/gcc-arm-embedded) has been superseeded by the new
GNU Arm Embedded one
(https://developer.arm.com/open-source/gnu-toolchain/gnu-rm

).

This also means a change of name from "GCC" to "GNU". Reflect this in
the enviroment variables so that the proper term is used henceforth.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>