FRV: Do some cleanups

1. frv doesn't support SMP, remove the useless SMP bits.

2. frv has its own alloc_task_struct, so define __HAVE_ARCH_TASK_STRUCT_ALLOCATOR
   (I am not sure if frv should use generic alloc_task_struct().)

Signed-off-by: WANG Cong <amwang@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>