The previous code had some errors that would be triggered on platforms
without stdatomics but with support for xadd assembly instruction.
The major error was combining two uint32_t values from the
multifield atomic structure using a logical AND '&&' instead of a
bitwise OR '|'.
Some preprocessor rules were redundant and thus were simplified,
regarding the definition of ISC_STATS_USEMULTIFIELDS macro.
Correctly changed rwlock type to read on isc_stats_get_counter.
8680c367cd