internal/syscall/unix: implement Eaccess on freebsd

Like on linux, use faccessat(AT_FDCWD, path, mode, AT_EACCESS)

Change-Id: I98c8af5008bfa7940abffa6fcb3766254955cb08
Reviewed-on: https://go-review.googlesource.com/c/go/+/531155
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/internal/syscall/unix/at_sysnum_freebsd.go

@@ -7,6 +7,8 @@ package unix
 import "syscall"
 
 const (
+	AT_EACCESS          = 0x100
+	AT_FDCWD            = -0x64
 	AT_REMOVEDIR        = 0x800
 	AT_SYMLINK_NOFOLLOW = 0x200
 

src/internal/syscall/unix/eaccess_freebsd.go

@@ -0,0 +1,26 @@
+// Copyright 2023 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.
+
+package unix
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+func faccessat(dirfd int, path string, mode uint32, flags int) error {
+	p, err := syscall.BytePtrFromString(path)
+	if err != nil {
+		return err
+	}
+	_, _, errno := syscall.Syscall6(syscall.SYS_FACCESSAT, uintptr(dirfd), uintptr(unsafe.Pointer(p)), uintptr(mode), uintptr(flags), 0, 0)
+	if errno != 0 {
+		return errno
+	}
+	return err
+}
+
+func Eaccess(path string, mode uint32) error {
+	return faccessat(AT_FDCWD, path, mode, AT_EACCESS)
+}

src/internal/syscall/unix/eaccess_other.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.
 
-//go:build unix && !linux
+//go:build unix && !freebsd && !linux
 
 package unix