x86/uv: memory allocation at initialization

Impact: on SGI UV platforms, fix boot crash

UV initialization is currently called too late to call alloc_bootmem_pages().
The current sequence is:

 start_kernel()
   mem_init()
     free_all_bootmem()           <--- discard of bootmem
   rest_init()
     kernel_init()
       smp_prepare_cpus()
       native_smp_prepare_cpus()
         uv_system_init()         <--- uses alloc_bootmem_pages()

It should be calling kmalloc().

Signed-off-by: Cliff Wickman <cpw@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>