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/fcntl.go

@@ -12,9 +12,8 @@ import "unsafe"
 // systems by fcntl_linux_32bit.go to be SYS_FCNTL64.
 var fcntl64Syscall uintptr = SYS_FCNTL
 
-// FcntlInt performs a fcntl syscall on fd with the provided command and argument.
-func FcntlInt(fd uintptr, cmd, arg int) (int, error) {
-	valptr, _, errno := Syscall(fcntl64Syscall, fd, uintptr(cmd), uintptr(arg))
+func fcntl(fd int, cmd, arg int) (int, error) {
+	valptr, _, errno := Syscall(fcntl64Syscall, uintptr(fd), uintptr(cmd), uintptr(arg))
 	var err error
 	if errno != 0 {
 		err = errno
@@ -22,6 +21,11 @@ func FcntlInt(fd uintptr, cmd, arg int) (int, error) {
 	return int(valptr), err
 }
 
+// FcntlInt performs a fcntl syscall on fd with the provided command and argument.
+func FcntlInt(fd uintptr, cmd, arg int) (int, error) {
+	return fcntl(int(fd), cmd, arg)
+}
+
 // FcntlFlock performs a fcntl syscall for the F_GETLK, F_SETLK or F_SETLKW command.
 func FcntlFlock(fd uintptr, cmd int, lk *Flock_t) error {
 	_, _, errno := Syscall(fcntl64Syscall, fd, uintptr(cmd), uintptr(unsafe.Pointer(lk)))