unix: fix redefinition of seek func on arm and gccgo

Change-Id: Ia23cb723766e9d76d51c24ace0157f47cb5b34ae
Reviewed-on: https://go-review.googlesource.com/c/sys/+/249617
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

unix/syscall_linux_arm.go

diff --git a/unix/syscall_linux_arm.go b/unix/syscall_linux_arm.go
index e1913e2..496837b 100644
--- a/unix/syscall_linux_arm.go
+++ b/unix/syscall_linux_arm.go
@@ -7,7 +7,6 @@
 package unix
 
 import (
-	"syscall"
 	"unsafe"
 )
 
@@ -49,10 +48,6 @@
 	return
 }
 
-// Underlying system call writes to newoffset via pointer.
-// Implemented in assembly to avoid allocation.
-func seek(fd int, offset int64, whence int) (newoffset int64, err syscall.Errno)
-
 func Seek(fd int, offset int64, whence int) (newoffset int64, err error) {
 	newoffset, errno := seek(fd, offset, whence)
 	if errno != 0 {

unix/syscall_linux_gc_arm.go

diff --git a/unix/syscall_linux_gc_arm.go b/unix/syscall_linux_gc_arm.go
new file mode 100644
index 0000000..8c514c9
--- /dev/null
+++ b/unix/syscall_linux_gc_arm.go
@@ -0,0 +1,13 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build arm,!gccgo,linux
+
+package unix
+
+import "syscall"
+
+// Underlying system call writes to newoffset via pointer.
+// Implemented in assembly to avoid allocation.
+func seek(fd int, offset int64, whence int) (newoffset int64, err syscall.Errno)