ext: Replace gzstream with iostream3 from zlib to avoid LGPL

This patch replaces the gzstream zlib wrapper with the iostream3
wrapper provided as part of zlib contributions. The main reason for
the switch is to avoid including LGPL in the default gem5
build. iostream3 is provided under a more permissive license:

The code is provided "as is", with the permission to use, copy,
modify, distribute and sell it for any purpose without fee.