gdb/gdbserver/

	gdb/gdbserver/
	* regcache.c (init_register_cache): Initialize
	regcache->register_status.
	(free_register_cache): Release regcache->register_status.
	(regcache_cpy): Copy register_status.
	(registers_to_string): Print 'x's for unavailable registers.
	(supply_register): Mark the register's status valid or
	unavailable, depending on whether a buffer was passed in or not.
	(supply_register_zeroed): New.
	(supply_regblock): Mark the registers' status valid or
	unavailable, depending on whether a buffer was passed in or not.
	* regcache.h (REG_UNAVAILABLE, REG_VALID): New defines.
	(struct regcache): New `register_status' field.
	(supply_register_zeroed): Declare.
	* i387-fp.c (i387_xsave_to_cache): Zero out registers using
	supply_register_zeroed, rather than passing a NULL buffer to
	supply_register.
	* tracepoint.c (fetch_traceframe_registers): Update comment.