util,python: Add check to ensure files are utf-8 in pre-commit

util,python: Add check to ensure files are utf-8 in pre-commit

The `file_from_index` function throws a UnicodeDecodeError if a modified
file targetted for style-checking (i.e. source-code) cannot be decoded
using `.decode("utf-8")`.

This check throws an error informing the user a submitted file must be
utf-8 encoded if this case arises.

Change-Id: I2361017f2e7413ed60f897d2301f2e4c7995dd76
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40015
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>