S390: Make IPA recognize tdescs with guarded storage

The in-process agent does not handle tdescs with guarded storage yet.
This is fixed.

gdb/gdbserver/ChangeLog:

	* linux-s390-ipa.c (get_ipa_tdesc): Add handling for
	S390_TDESC_GS.
	* linux-s390-low.c (s390_get_ipa_tdesc_idx): Likewise.
	(initialize_low_tracepoint): Call init_registers_s390x_gs_linux64
	and init_registers_s390_gs_linux64.