Work around an MSVC bug

The assembly code generated by MSVC for at least some signed comparisons involving atomic variables incorrectly uses unsigned conditional jumps instead of signed ones. In particular, the checks in isc_log_wouldlog() are affected in a way which breaks logging on Windows and thus also all system tests involving a named instance. Work around the issue by assigning the values returned by atomic_load_acquire() calls in isc_log_wouldlog() to local variables before performing comparisons.

Closes #1742 (closed)