add linux amd64 build tags to the lxc driver

This makes it possible to avoid building the lxc driver on anything which isn't linux amd64. This change is important because Docker doesn't need to build `kr/pty` for lxc on unsupported platforms.

```
# github.com/docker/docker/pkg/term
pkg/term/term.go:16: undefined: Termios
# github.com/docker/docker/pkg/system
pkg/system/stat_unsupported.go:7: undefined: syscall.Stat_t
pkg/system/stat_unsupported.go:11: undefined: syscall.Stat_t
# github.com/kr/pty
vendor/src/github.com/kr/pty/ioctl.go:6: undefined: syscall.SYS_IOCTL
vendor/src/github.com/kr/pty/ioctl.go:6: not enough arguments in call to syscall.Syscall
vendor/src/github.com/kr/pty/run.go:21: unknown syscall.SysProcAttr field 'Setctty' in struct literal
vendor/src/github.com/kr/pty/run.go:21: unknown syscall.SysProcAttr field 'Setsid' in struct literal
vendor/src/github.com/kr/pty/util.go:26: undefined: syscall.SYS_IOCTL
vendor/src/github.com/kr/pty/util.go:28: undefined: syscall.TIOCGWINSZ
vendor/src/github.com/kr/pty/util.go:30: not enough arguments in call to syscall.Syscall
```
ends up looking like this after the PR
```
# github.com/docker/docker/pkg/term
pkg/term/term.go:16: undefined: Termios
# github.com/docker/docker/pkg/system
pkg/system/stat_unsupported.go:7: undefined: syscall.Stat_t
pkg/system/stat_unsupported.go:11: undefined: syscall.Stat_t
```