libc: properly forward the compat syscall references to libsys

same as it was done for setgroups@FBSD_1.0.
Switch from weakref to symver, since GNU as cannot handle version spec
with weakref.

Reviewed by: olce
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D52687