gdb/x86: Implement ax_pseudo_register_collect hook.

Makes "collect $ymm15" action work.

gdb/ChangeLog:

	* amd64-tdep.c (amd64_ax_pseudo_register_collect): New function.
	(amd64_init_abi): Fill ax_pseudo_register_collect hook.
	* gdb/i386-tdep.c (i386_pseudo_register_read_into_value): Remove
	misleading comment.
	(i386_pseudo_register_write): Ditto.
	(i386_ax_pseudo_register_collect): New function.
	(i386_gdbarch_init): Fill ax_pseudo_register_collect hook.
	* i386-tdep.h: Add i386_ax_pseudo_register_collect prototype.