[ELF] splitNonStrings: switch to xxh3_64bits

[ELF] splitNonStrings: switch to xxh3_64bits

This is primarily used for .rodata.cst* duplicate elimination. The
sections are usually much smaller than .debug_str (D154813), so the
speedup is negligible. We do this switch for consistency as we want to
eliminate xxh64 in lld.