libc/{i386,amd64}: Do not export .cerror when building WITHOUT_SYMVER

Further to r240152 (i386) and r240178 (amd64), hide the .cerror symbol
so that it is not exported if symbol versioning is not in use. Without
this change WITHOUT_SYMVER libc contains .text relocations for .cerror,
as described in LLVM PR 26813 (http://llvm.org/pr26813).

This is a no-op for the regular build as the symbol version script
already controls .cerror visibility.

PR: 207712
Submitted by: Rafael Espíndola
Reviewed by: jilles, kib
Differential Revision: https://reviews.freebsd.org/D5571