clang: allow ifunc resolvers to accept arguments

Previously Clang required ifunc resolution functions to take no
arguments, presumably because GCC documented ifunc resolvers as taking
no arguments. However, GCC accepts resolvers accepting arguments, and
our rtld passes CPU ID information (cpuid, hwcap, etc.) to ifunc
resolvers. Just remove the check from the in-tree compiler for our in-
tree compiler; a different (per-OS) approach may be required upstream.

Reported by: mjg
Approved by: re (rgrimes)
MFC after: 1 week
Relnotes: Yes
Sponsored by: The FreeBSD Foundation