S390: Enable "maint set show-debug-regs"

S390: Enable "maint set show-debug-regs"

Implement a new function for dumping the S390 "debug
registers" (actually, the PER info) and invoke it at appropriate places.
Respect the variable show_debug_regs and make it settable by the user.

gdb/ChangeLog:

	* s390-linux-nat.c (gdbcmd.h): New include.
	(s390_show_debug_regs): New function.
	(s390_stopped_by_watchpoint): Call it, if show_debug_regs is set.
	(s390_prepare_to_resume): Likewise.
	(_initialize_s390_nat): Register the command "maint set
	show-debug-regs".