Move Linux/ppc64 specific cpu_online() to os/os-linux.h

Move Linux/ppc64 specific cpu_online() to os/os-linux.h

cpu_online() implementation for ppc64 added to os/os.h by c5dd5d97
(powerpc: fix cpus_online() to get correct max CPU number for powerpc64)
seems Linux specific, thus it should be moved to os/os-linux.h.

For example, non standard sysconf values _SC_NPROCESSORS_ONLN and
_SC_NPROCESSORS_CONF do the same thing with libc in FreeBSD, whereas
they aren't always the same with glibc. This implies c5dd5d97 is about
Linux, and wouldn't be a general solution that needs to be in os/os.h.

sysconf(3) in FreeBSD
https://github.com/freebsd/freebsd/blob/master/lib/libc/gen/sysconf.c#L588

Another possibility is AIX in addition to Linux, but below website
has an example of both _SC_NPROCESSORS_ONLN and _SC_NPROCESSORS_CONF
having the same value under SMT.

Topic: prtconf and sysconf produce different numbers of processors
https://www.ibm.com/developerworks/community/forums/html/topic?id=77777777-0000-0000-0000-000014250083
Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@fb.com>