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>