y2038: Introduce struct __ntptimeval64 - new internal glibc type

This type is a glibc's "internal" type to get time parameters data from
Linux kernel (NTP daemon interface). It stores time in struct __timeval64
rather than struct timeval, which makes it Y2038-proof.

Build tests:
./src/scripts/build-many-glibcs.py glibcs

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