llvm: Update build system to support toolchain

This change introduces the building of the SCP firmware
using the LLVM toolchain (https://llvm.org

).
Currently the support is enabled for the target
architecture ARMv7-M and the Newlib library but it can
easily modified for other architecture targets and
standard C libraries.

Change-Id: I980b6b84876f922dcc87cbd97a8c2505a6e308fa
Signed-off-by: Luca Vizzarro <Luca.Vizzarro@arm.com>