Issue #17615: On Windows (VS2010), Performances of wmemcmp() to compare Unicode

strings are not convincing. For UCS2 (16-bit wchar_t type), use a dummy loop
instead of wmemcmp(). The dummy loop is as fast, or a little bit faster.

wchar_t is only 16-bit long on Windows. wmemcmp() is still used for 32-bit
wchar_t.