Linux: Move timer_create, timer_delete from librt to libc

The symbols were moved using scripts/move-symbol-to-libc.py.

timer_create and timer_delete are tied together via the int/timer_t
compatibility code.  The way the ABI intransition is implemented
is changed with this commit: the implementation is now consolidated
in one file with a TIMER_T_WAS_INT_COMPAT check.

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