doc: Expand doc on 3rd-party cross compilers

Add documentation for OS X users that want to compile
using an available 3rd-party cross compiler and also
complete the section to describe the existing support
for the GCC ARM Embedded toolchain.

Change-Id: I97d27d46c0048a49029dbe4d66fe141755c226e9
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>