config: Add a system clock command-line option

This patch adds a 'sys_clock' command-line option and use it to assign
clocks to the system during instantiation.

As part of this change, the default clock in the System class is
removed and whenever a system is instantiated a system clock value
must be set. A default value is provided for the command-line option.

The configs and tests are updated accordingly.

configs/example/memtest.py

@@ -144,6 +144,7 @@ for scale in treespec[:-2]:
 system = System(funcmem = SimpleMemory(in_addr_map = False),
                 funcbus = NoncoherentBus(),
                 physmem = SimpleMemory(latency = "100ns"))
+system.clock = options.sys_clock
 
 def make_level(spec, prototypes, attach_obj, attach_port):
      fanout = spec[0]