gdb: make timestamped_file implement can_emit_style_escape

gdb: make timestamped_file implement can_emit_style_escape

In our AMDGPU downstream port, we use styling in some logging output.
We noticed it stopped working after the gdb_printf changes.  Making
timestamped_file implement can_emit_style_escape (returning the value of
the stream it wraps) fixes it.  To show that it works, modify some
logging statements in auto-load.c to output style filenames.  You can
see it in action by setting "set debug auto-load 1" and running a
program.  We can incrementally add styling to other debug statements
throughout GDB, as needed.

Change-Id: I78a2fd1e078f80f2263251cf6bc53b3a9de9c17a