bootmem: Add alloc_bootmem_align()

Add an alloc_bootmem_align() interface to allocate bootmem with
specified alignment.  This is necessary to be able to allocate the
xsave area in a subsequent patch.

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
LKML-Reference: <20101116212441.977574826@sbsiddha-MOBL3.sc.intel.com>
Acked-by: H. Peter Anvin <hpa@linux.intel.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: <stable@kernel.org>