y2038: alpha: Rename valid_timeval_to_timeval64 to valid_timeval32_to_timeval

Without this patch the naming convention for functions to convert
struct timeval32 to struct timeval (which supports 64 bit time on Alpha) was
a bit misleading. The name 'valid_timeval_to_timeval64' suggest conversion
of struct timeval to struct __timeval64 (as in ./include/time.h).

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

Hence the helper function naming change to 'valid_timeval32_to_timeval'.

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>