unix: unify definition of func fcntl

For linux and *bsd, implement func fcntl in fcntl.go instead of
generating it. Implement it using fcntl64Syscall, so SYS_FCNTL64 is used
on 32-bit linux.

For aix, darwin and solaris continue generate func fcntl as before.

Change-Id: I3750b88d1ee5573daaee2b75c369f723fde8834a
Reviewed-on: https://go-review.googlesource.com/c/sys/+/212337
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_bsd.go

@@ -577,8 +577,6 @@ func Futimes(fd int, tv []Timeval) error {
 	return futimes(fd, (*[2]Timeval)(unsafe.Pointer(&tv[0])))
 }
 
-//sys	fcntl(fd int, cmd int, arg int) (val int, err error)
-
 //sys   poll(fds *PollFd, nfds int, timeout int) (n int, err error)
 
 func Poll(fds []PollFd, timeout int) (n int, err error) {