x86-32: Align IRQ stacks properly

As suggested by Steven Rostedt we need to align the IRQ stacks to the
stack size, not just the page size to make them work for stack traces
and other things that depend on finding the stack slot itself with 8k
stacks.

Signed-off-by: Christoph Hellwig <hch@lst.de>
LKML-Reference: <20100727121313.GA19976@lst.de>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>