readme: explain how to run benchmarks with readfile straight out

gem5 cache automate benchmarks, and remove results that were scattered all over.

Move console fun under linux kernel section, since not important enough
for getting started.

Set arch x86_64 by default through common on all scripts.

trace-boot

@@ -2,7 +2,6 @@
 set -eu
 . common
 set -- ${cli_trace_boot:-} "$@"
-arch=x86_64
 while getopts a: OPT; do
   case "$OPT" in
     a)