util: Move source files into a src directory.

util: Move source files into a src directory.

This also moves the Makefiles, which will still produce build output in
the src directory. This is to prepare for a scons based build system
which will create a separate build directory with build artifacts.

Change-Id: I7c6d325e1d0a428656b2e3070b5e942515543696
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27209
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>