y2038: alpha: Rename valid_timeval64_to_timeval to valid_timeval_to_timeval32

The name 'valid_timeval64_to_timeval' suggest conversion of struct
__timeval64 to struct timeval (as in ./include/time.h).

As on the alpha the struct timeval supports 64 bit time, it seems more
feasible to emphasis struct timeval32 in the conversion function name.

Hence the helper function naming change to 'valid_timeval_to_timeval32'.

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>