sgi/install_prerequisites.sh: do toolchain installation only for x86_64

The architecture of the host machine is checked in the script and gcc
toolchain will be installed only for x86_64 host architecture. For
Aarch64 machines, native compiler arm-none-eabi-gcc is pre-installed.

Change-Id: If7726e6cddd4f44b98fa8aac5a6f60b2e8b59f7f
Signed-off-by: Tony K Nadackal <tony.nadackal@arm.com>