Arguments: Get rid of duplicate code for the Arguments class in each architecture.

Arguments: Get rid of duplicate code for the Arguments class in each architecture.
Move the argument files to src/sim and add a utility.cc file with a function
getArguments() that returns the given argument in the architecture specific fashion.
getArguments() was getArg() is the architecture specific Argument class and has had
all magic numbers replaced with meaningful constants. Also add a function to the
Argument class for testing if an argument is NULL.

--HG--
rename : src/arch/alpha/arguments.cc => src/sim/arguments.cc
rename : src/arch/alpha/arguments.hh => src/sim/arguments.hh
extra : convert_revision : 8b93667bafaa03b52aadb64d669adfe835266b8e