util: Explicitly decode/encode in utf-8.

util: Explicitly decode/encode in utf-8.

The default encoding for python 2 is ascii which can't handle some
characters in, for instance, people's names which have accented letters.
This change explicitly selects the utf-8 encoding which pacifies python
and is mostly equivalent except in these rare cases.

In python 3, the default encoding is utf-8 to begin with, and it's no
longer possible to change it. In this case, explicitly selecting the
encoding is redundant but harmless.

When we support only python 3, then this change can be reverted.

Thanks to Lakin Smith for proposing a related solution and pointing out
some information that led to this one.

Change-Id: I99bd59063c77edd712954ffe90d7de320ade49ea
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33575
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Lakin Smith <lakindsmith@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>