unix: Added posix_fadvise support for Linux and FreeBSD

Support includes 386, amd64, and arm for both Linux and FreeBSD.
FreeBSD changes courtesy Dave Chapeskie <dchapeskie@gmail.com>

Change-Id: I89dc18533d7178e74ae6c442a2e6272c9bd9692c
Reviewed-on: https://go-review.googlesource.com/8055
Reviewed-by: Rob Pike <r@golang.org>

unix/syscall_linux.go

@@ -799,6 +799,7 @@ func Mount(source string, target string, fstype string, flags uintptr, data stri
 //sys	EpollWait(epfd int, events []EpollEvent, msec int) (n int, err error)
 //sys	Exit(code int) = SYS_EXIT_GROUP
 //sys	Faccessat(dirfd int, path string, mode uint32, flags int) (err error)
+//sys	Fadvise(fd int, offset int64, length int64, advice int) (err error) = SYS_FADVISE64
 //sys	Fallocate(fd int, mode uint32, off int64, len int64) (err error)
 //sys	Fchdir(fd int) (err error)
 //sys	Fchmod(fd int, mode uint32) (err error)
@@ -930,7 +931,6 @@ func Munmap(b []byte) (err error) {
 // EpollWaitOld
 // Eventfd
 // Execve
-// Fadvise64
 // Fgetxattr
 // Flistxattr
 // Fork