util: Convert the m5 utility to C++.

util: Convert the m5 utility to C++.

This will make it possible to use the googletest unit testing framework,
and will let us use c++ mechanisms to simplify and streamline the code.

Change-Id: I8ab358de47ce6b5c2d601cc0b9f2a694b2037a9b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27548
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabeblack@google.com>