libc: stop exporting cerror

i386 stopped exporting .cerror in r240152, and likewise for amd64 in
r240178. It is not used by other libraries on any platform, so apply
the same change to the remaining architectures.

Reviewed by: jhibbits, jilles
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D5774