arm64 libc: hide .cerror, .curbrk, .minbrk for WITHOUT_SYMVER

When symver is in use these are hidden because they're not listed in
the Symbol.map. Add an explicit .hidden so they are also hidden in the
WITHOUT_SYMVER case.

Reviewed by: andrew
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D5775