MpactCheriotCPU: track new simulator functionality

- load_elf replaces load_executable to support passing symbols into
  the simulator
- symbols can be used in the cli in lieu of memory addresses
- if symbols are passed to the simulator and the "tohost" symbol is
  defined, the simulator will plant a watchpoint to identify when a
  simulation has terminated
- (unrelated) WaitForCLI now defaults to true

Note: this requires an updated cheriot toolchain

Bypass-Presubmit-Reason: no sencha tests

Change-Id: I890c5332993080e7bec2a3efdc209af029d6f538