Change (blank)cpu affinity macros to inline functions

gcc warns -Wunused-value on some environments (e.g. BSD) when
the following cpu affinity macros are used in non conditional code.

Also removed #ifdef FIO_HAVE_CPU_AFFINITY in gettime-thread.c
since this variable needs to be visible when calling the function.

gettime-thread.c: In function 'gtod_thread_main':
os/os.h:82:36: warning: statement with no effect [-Wunused-value]
 #define fio_setaffinity(pid, mask) (0)
                                    ^
gettime-thread.c:48:2: note: in expansion of macro 'fio_setaffinity'
  fio_setaffinity(gettid(), fio_gtod_cpumask);