btrace, gdbserver: Add ptwrite to btrace_config_pt.

This enables gdb and gdbserver to communicate about ptwrite support.  If
ptwrite support would be enabled unconditionally, GDBs with older libipt
versions would break.

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