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>

src/python/m5/main.py

@@ -273,6 +273,7 @@ def main(*args):
         done = True
         print('Build information:')
         print()
+        print('gem5 version %s' % defines.gem5Version)
         print('compiled %s' % defines.compileDate)
         print('build options:')
         keys = list(defines.buildEnv.keys())
@@ -336,6 +337,7 @@ def main(*args):
         print(brief_copyright)
         print()
 
+        print("gem5 version %s" % defines.gem5Version)
         print("gem5 compiled %s" % defines.compileDate)
 
         print("gem5 started %s" %