y2038: Define __suseconds64_t type to be used with struct __timeval64

The __suseconds64_t type is supposed to be the 64 bit type across all
architectures.

It would be mostly used internally in the glibc - however, when passed to
Linux kernel (very unlikely), if necessary, it shall be converted to 32
bit type (i.e. __suseconds_t)

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>