CheckerCPU: Make CheckerCPU runtime selectable instead of compile selectable

Enables the CheckerCPU to be selected at runtime with the --checker option
from the configs/example/fs.py and configs/example/se.py configuration
files.  Also merges with the SE/FS changes.

src/arch/SConscript

@@ -94,8 +94,7 @@ isa_parser = File('isa_parser.py')
 # autogenerated files as targets and isa parser itself as a source.
 def isa_desc_emitter(target, source, env):
     cpu_models = list(env['CPU_MODELS'])
-    if env['USE_CHECKER']:
-        cpu_models.append('CheckerCPU')
+    cpu_models.append('CheckerCPU')
 
     # Several files are generated from the ISA description.
     # We always get the basic decoder and header file.