Use atomic operations whenever available

InnoDB up to MariaDB 10.1 defines the compile-time constant UNIV_WORD_SIZE incorrectly on 64-bit Windows systems as 4:

#ifdef _WIN32

#define UNIV_WORD_SIZE          4

#elif defined(_WIN64)

#define UNIV_WORD_SIZE          8

#else

/** MySQL config.h generated by GNU autoconf will define SIZEOF_LONG in Posix */

#define UNIV_WORD_SIZE          SIZEOF_LONG

#endif

This error has been corrected in MariaDB Server 10.2. The problem is that _WIN32 is defined also on 64-bit Windows, causing UNIV_WORD_SIZE to be defined as the incorrect value 4 instead of 8.

Furthermore, the logic around updating some file counters is refusing to use atomic operations on 32-bit systems, and the variables os_n_pending_reads, os_n_pending_writes are redundant.