libatomic: Add support for __ifunc_arg_t arg in ifunc resolver

With support for new atomic features in Armv9.4-a being indicated by
HWCAP2 bits, Libatomic's ifunc resolver must now query its second
argument, of type __ifunc_arg_t*.

We therefore make this argument known to libatomic, allowing us to
query hwcap2 bits in the following manner:

  bool
  resolver (unsigned long hwcap, const __ifunc_arg_t *features);
  {
    return (features->hwcap2 & HWCAP2_<FEAT_NAME>);
  }

libatomic/ChangeLog:

	* config/linux/aarch64/host-config.h (__ifunc_arg_t):
	Conditionally-defined if `sys/ifunc.h' not found.
	(_IFUNC_ARG_HWCAP): Likewise.
	(IFUNC_COND_1): Pass __ifunc_arg_t argument to ifunc.
	(ifunc1): Modify function signature to accept __ifunc_arg_t
	argument.
	* configure.tgt: Add second `const __ifunc_arg_t *features'
	argument to IFUNC_RESOLVER_ARGS.