gem5: track build and run variants separately with -M and -N

Otherwise, checking out branches is too insane, as it does not
update the worktrees, even though the gem5/gem5 module was updated.

gem5: expose build types, document debug builds.

simultaneous runs: store stdout and stderr on a file to allow running
all from a single terminal on the background cleanly.

gem5-bench-cache

@@ -68,7 +68,7 @@ if "$generate_checkpoints"; then
   # Create the checkpoints after the kernel boot.
   rm -rf "${common_m5out_dir}"/cpt.*;
   printf 'm5 exit' > "${readfile_file}"
-  cpt_cmd="-E 'm5 checkpoint;m5 readfile > a.sh;sh a.sh'"
+  cpt_cmd="-E '/gem5.sh'"
   # 1
   ./eeval "$cmd $cpt_cmd"
   # RESTORE_INVESTIGATION