gem5/gem5: upgrade to 20.1.0.5

This is the latest point release in the 20.1 series, and also fixes the build with
scons 4.1 (which is now in master).

Change-Id: I31b8aabc8af47cc64af68f2fd0ee2b9b9b656e89
Signed-off-by: Ross Burton <ross.burton@arm.com>