rcutorture: Refactor to enable non-x86 architectures

This commit expands the checks for what architecture is running to generate
additional qemu-system- commands, then uses the resulting qemu-system-
command name to choose different qemu arguments as needed for different
architectures.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Greg KH <gregkh@linuxfoundation.org>