kubectl's dir should be 386 not x86 in x86 enviroment

Before applying this patch, `cluster/kubectl.sh` failed to find
`kubectl` binary in x86 environment.

e.g. The latest release v0.17.1 contains following binaries:
https://github.com/GoogleCloudPlatform/kubernetes/releases/tag/v0.17.1

```
$ tree platforms/
platforms/
├── darwin
│   ├── 386
│   │   └── kubectl
│   └── amd64
│       └── kubectl
├── linux
│   ├── 386
│   │   └── kubectl
│   ├── amd64
│   │   └── kubectl
│   └── arm
│       └── kubectl
└── windows
    └── amd64
            └── kubectl.exe
```

cluster/kubectl.sh

@@ -59,7 +59,7 @@ case "$(uname -m)" in
     host_arch=arm
     ;;
   i?86*)
-    host_arch=x86
+    host_arch=386
     ;;
   *)
     echo "Unsupported host arch. Must be x86_64, 386 or arm." >&2