Add os/os-linux-syscall.h to separate syscall NR from arch headers

Linux syscall NR should be defined under os/ with ifdef(arch)
switch rather than arch/ where headers are included not only by
Linux but also all other supported OS that don't need these NR.
(It'll get worse and worse if someone wants to add something
similar under arch/ for other supported OS)

Not sure if you like the way I separated these by adding
os/os-linux-syscall.h, but doing something like
 #if FIO_OS is (Linux or Android)
 ...
 #endif
in arch/arch-*.h doesn't work because arch headers are to be
included prior to os/os-*.h.

Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>