parsec: general cleanup, retest everything

Get rid of br2_parsec, since there are just too many possible variations on that file.

Blow up the gem5 vs qemu table to make it saner, add missing aarch64 entries.

Make sections titles describing number of cores and memory size emulator agnostic.

run-usage.adoc

@@ -35,6 +35,9 @@
 |`-K` |              |Use KVM. Only works if guest arch == host arch.
 |`-k` |              |Enable KGDB.
 |`-m` |              |Set the memory size of the guest. E.g.: `-m 512M`. Default: `256M`.
+                      The default is the minimum ammount that boots all archs without extra
+                      options added. Anything lower will lead some arch to fail to boot.
+                      Any
 |`-T` |              |Enable extra QEMU trace events.
                       `./configure --enable-trace-backends=simple` seems to enable
                       some by default, e.g. `pr_manager_run`, and I don't know how to