aarch64: simplify the DT_AARCH64_VARIANT_PCS handling code

Remove unnecessary variant_pcs field: the dynamic tag can be checked
directly.

	* sysdeps/aarch64/dl-machine.h (elf_machine_runtime_setup): Remove the
	DT_AARCH64_VARIANT_PCS check.
	(elf_machine_lazy_rel): Use l_info[DT_AARCH64 (VARIANT_PCS)].
	* sysdeps/aarch64/linkmap.h (struct link_map_machine): Remove
	variant_pcs.