define uint64_t type grib_en/decode and use them

[mtasaka]

Currently grib_encode_size_t / grib_decode_size_t are defined and especially on windows where size_t is 64 bit, encode_double_array and decode_double_array use them.

So especially for 32 bit linux system, define uint64_t type grib_encode / grib_decode and use them instead of size_t type ones. This method is more portable.

The proposal patch fixes the following test failure on Linux 32 bit:

#  35 - eccodes_t_bufr_ecc-1290

Forwarded from: https://jira.ecmwf.int/browse/SUP-3561