armclang: support for armclang compiler and armlink linker

This is the initial support for the armclang compiler together with the
armlink linker.

Introduced in this commit:
- armclang compiler support
- armlink linker support
- armlink scatter file generator for scatter loading
- dual pass linker script generation

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>