rt: avoid PLT setup in timer_[sg]ettime

The functions __timer_gettime64 and __timer_settime64 live in librt, not
libc.  Use proper hidden aliases so that the callers do not need to set up
the PLT register.

Fixes commits cae1635a ("y2038: linux: Provide __timer_settime64
implementation") and 562cdc19 ("y2038: linux: Provide __timer_gettime64
implementation").