base: Rework the trie dump function to accept a different ostream.

base: Rework the trie dump function to accept a different ostream.

It might often be useful to write output to cout when dumping a trie,
but sometimes it might be useful to dump ot to something else like a
string stream instead.

Change-Id: Iaa4ae772c902b7dbc753f320d1a7eb5fcd4a3db3
Reviewed-on: https://gem5-review.googlesource.com/6266
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Maintainer: Gabe Black <gabeblack@google.com>