cpu: Add header files for checker CPUs

cpu: Add header files for checker CPUs

In order to create reliable SWIG wrappers, we need to include the
declaration of the wrapped class in the SWIG file. Previously, we
didn't expose the declaration of checker CPUs. This patch adds header
files for such CPUs and include them in the SWIG wrapper.

--HG--
rename : src/cpu/dummy_checker_builder.cc => src/cpu/dummy_checker.cc
rename : src/cpu/o3/checker_builder.cc => src/cpu/o3/checker.cc