internal/syscall/unix: use libc based fcntl for IsNonblock on aix and solaris

On aix and solaris (like on darwin) use libc fcntl to implement
IsNonblock instead of Syscall(SYS_FCNTL, ...) which isn't supported.

Change-Id: I989b02aa0c90b7e2dae025572867dda277fef8be
Reviewed-on: https://go-review.googlesource.com/c/go/+/212600
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/internal/syscall/unix/nonblocking.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build aix dragonfly freebsd linux netbsd openbsd solaris
+// +build dragonfly freebsd linux netbsd openbsd
 
 package unix
 

src/internal/syscall/unix/nonblocking_darwin.go → src/internal/syscall/unix/nonblocking_libc.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build darwin
+// +build aix darwin solaris
 
 package unix
 
@@ -19,6 +19,6 @@ func IsNonblock(fd int) (nonblocking bool, err error) {
 	return flag&syscall.O_NONBLOCK != 0, nil
 }
 
-// Implemented in syscall/syscall_darwin.go.
+// Implemented in the syscall package.
 //go:linkname fcntl syscall.fcntl
 func fcntl(fd int, cmd int, arg int) (int, error)