arm-bsp, gem5-arm64: Update to v20 version

This patch updates gem5-aarch64-bootloader and gem5-aarch64-native to
the release tag v20.0.0.1.

Also apply populate the right compiler and linker flags to get
gem5-aarch64-native properly building

Change-Id: Icc2203163105373cf030975c1b12f1d4f2fcb03c
Issue-Id: SCM-1014
Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>