sysdeps/clock_gettime: Use clock_gettime64 if avaliable

With the clock_gettime64 call we prefer to use vDSO. There is no call
to clock_gettime64 on glibc with older headers and kernel 5.1+ if it
doesn't support vDSO.

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