gdb: make gdb_printing_disassembler::stream public

In the ROCm port, we need to access the underlying stream of a
gdb_printing_disassembler, so make it public.  The reason we need to
access it is to know whether it supports style escape code.  We then
pass that information to a temporary string_file we use while
symbolizing addresses.

Change-Id: Ib95755a4a45b8f6478787993e9f904df60dd8dc1
Approved-By: Andrew Burgess <aburgess@redhat.com>