y2038: Introduce struct __utimbuf64 - new internal glibc type

This type is a glibc's "internal" type to store file's access and modification
times in __time64_t rather than __time_t, which makes it Y2038-proof.

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

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