Use aarch32 toolchain from Arm Developer

The SCP component is requiring an older version than the one found in
standard Ubuntu repositories, thus this update aligns the installation
and build system to use the toolchain from Arm Developer website in both
supported build architectures.

Signed-off-by: Nuno Lopes <nuno.lopes@arm.com>
Change-Id: Ida9cebe396e257828fbb500ee693a23d8a5e50ea