gdbserver/s390: Advertise Z0 packet support.

gdbserver/s390: Advertise Z0 packet support.

This is necessary for upcoming tracepoint support - otherwise, setting
a tracepoint and a breakpoint on the same address will fail, since gdbserver
won't know about gdb's breakpoint.

Tested on s390x-ibm-linux-gnu and s390-ibm-linux-gnu, RHEL 7.2.

gdb/gdbserver/ChangeLog:

	* linux-s390-low.c (s390_supports_z_point_type): New function.
	(struct linux_target_ops): Wire s390_supports_z_point_type in.