internal/syscall/unix: don't use linkname to refer to syscall.fcntl

Just open-code the fcntl syscall instead of relying on the obscurity of
go:linkname.

Change-Id: I3e4ec9db6539e016f56667d7b8b87aa37671d0e7
Reviewed-on: https://go-review.googlesource.com/130736
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

@@ -6,18 +6,12 @@
 
 package unix
 
-import (
-	"syscall"
-	_ "unsafe" // for go:linkname
-)
-
-//go:linkname syscall_fcntl syscall.fcntl
-func syscall_fcntl(fd int, cmd int, arg int) (val int, err error)
+import "syscall"
 
 func IsNonblock(fd int) (nonblocking bool, err error) {
-	flag, err := syscall_fcntl(fd, syscall.F_GETFL, 0)
-	if err != nil {
-		return false, err
+	flag, _, e1 := syscall.Syscall(syscall.SYS_FCNTL, uintptr(fd), uintptr(syscall.F_GETFL), 0)
+	if e1 != 0 {
+		return false, e1
 	}
 	return flag&syscall.O_NONBLOCK != 0, nil
 }