get_source.sh: updates for 32bit arm targets

Updates get_source.sh common script to support fetch sources for a 32bit
arm target in which case we should fetch Linaro's 32bit cross toolchain
and not consider BSA/SBSA tests and Linux kernel patches that are
applicable only to Aarch64 targets.

Signed-off-by: Christophe Priouzeau <christophe.priouzeau@foss.st.com>
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>