util: Pull "usage()" out of the call types in the m5 utility.

Also pull common implementations of some call type methods into the base
class, and make disappearing call types clean themselves up to make the
test a little simpler and less error prone.

Change-Id: Ie178fe02d41587647ddc90a084d1d1142b84dde9
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27687
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>

util/m5/src/SConscript.native

@@ -28,7 +28,7 @@ Import('*')
 env.Append(CPPPATH=Dir('.'))
 
 env.GTest('args')
-env.GTest('call_type')
+env.GTest('call_type', 'call_type.test.cc', 'call_type.cc', 'args.cc')
 env.GTest('command', 'command.test.cc', 'command.cc', 'args.cc')
 
 command_tests = env.SConscript('command/SConscript.native', exports='env')