Allow trailing whitespace in str_to_size_t
Fixes #6984, first reported by sqrmax on the Discord, causing incorrect detection of NUMA and L3 domains on Linux due to trailing whitespace when reading from sysfs.
This was introduced by #6844 which made `str_to_size_t` too strict, since `std::stoull` allows trailing characters (and leading whitespace, although this part is not relevant for us).
Before:
$ ./bins/stockfish.master speedtest
Stockfish dev-20260716-ebcea3ef by the Stockfish developers (see AUTHORS file)
info string Using 224 threads with NUMA node thread binding: 64/32:64/32:64/32:32/16
After:
$ ./stockfish speedtest
Stockfish dev-20260718-m-cb702902 by the Stockfish developers (see AUTHORS file)
info string Using 224 threads with NUMA node thread binding: 32/32:32/32:32/32:32/32:32/32:32/32:32/32
closes https://github.com/official-stockfish/Stockfish/pull/6989
No functional change