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>
