Linux: Move timer_gettime, __timer_gettime64 from librt to libc

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

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>