S390: Add guarded-storage register support to GDB

S390: Add guarded-storage register support to GDB

Recognize targets with the new guarded-storage feature and then present
the guarded-storage registers and the Linux-specific guarded-storage
broadcast control block appropriately.

gdb/ChangeLog:

	* s390-linux-nat.c (have_regset_gs): New static variable.
	(s390_linux_fetch_inferior_registers): Handle guarded-storage
	control block and guarded-storage broadcast control regsets.
	(s390_read_description): Detect whether the target has
	guarded-storage support, return appropriate tdesc.
	* s390-linux-tdep.c (features/s390-gs-linux64.c): New include.
	(features/s390x-gs-linux64.c): Likewise.
	(struct gdbarch_tdep) <have_gs>: New field.
	(s390_regmap_gs, s390_regmap_gsbc, s390_gs_regset)
	(s390_gsbc_regset): New variables.
	(s390_iterate_over_regset_sections): Iterate over s390_gs_regset
	and s390_gsbc_regset, if applicable.
	(s390_core_read_description): Check whether core file was from a
	target with guarded-storage support; include appropriate regsets.
	(s390_gdbarch_init): Add registers for guarded-storage support.
	(_initialize_s390_tdep): Initialize new target descriptions that
	include registers for guarded-storage support.
	* s390-linux-tdep.h (HWCAP_S390_GS, S390_GSD_REGNUM)
	(S390_GSSM_REGNUM, S390_GSEPLA_REGNUM)
	(S390_BC_GSD_REGNUM, S390_BC_GSSM_REGNUM): New defines.
	(S390_NUM_REGS): Adjust macro definition.
	(s390_gs_regset, s390_gsbc_regset, tdesc_s390_gs_linux64)
	(tdesc_s390x_gs_linux64): New declarations.