y2038: Provide conversion helpers for struct __timeval64

Those functions allow easy conversion between Y2038 safe, glibc internal
struct __timeval64 and other time related data structures (like struct timeval
or struct __timespec64).

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

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>