Linux: Move timer helper routines from librt to libc

This adds several temporary GLIBC_PRIVATE exports.  The symbol names
are changed so that they all start with __timer_.

It is now possible to invoke the fork handler directly, so
pthread_atfork is no longer necessary.  The associated error cannot
happen anymore, and cancellation handling can be removed from
the helper thread routine.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>