arm-bsp: Switch to gem5 20.0 staging version

Switch gem5 to use current HEAD version of the staging 20.0 branch to
solve compilation issues related to the move to python3

Create a link named python to python3 as some of gem5 python scripts
are looking for python and are compatible with both python2 and
python3.

Change-Id: I59256a8a6783a9614f5a2d5d760fada5bd6d0337
Signed-off-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>