base,misc: Added version to code

base,misc: Added version to code

This version ID string can be accessed from elsewhere in the CPP
codebase using `extern const char *gem5Version;`. The python variable
"gem5Version" may be accessed in by importing the generated
"defines.py". When gem5 now runs it will output the version
currently being used to the CLI.

Change-Id: I4feb6b7317cd2922c56eb8ce3e532b0bfa0e6ed3
Issue-on: https://gem5.atlassian.net/browse/GEM5-411
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27787
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>