btrace: Extend ptwrite event decoding.

Call the ptwrite filter function whenever a ptwrite event is decoded.
The returned string is written to the aux_data string table and a
corresponding auxiliary instruction is appended to the function segment.

Approved-By: Markus Metzger <markus.t.metzger@intel.com>
Reviewed-By: Eli Zaretskii <eliz@gnu.org>