x86 transition to 4k stacks (0/3)

The kernel currently uses an 8k stack, per task.  Here is the 
infrastructure needed to allow us to halve that at some point in the 
future.

This is a port of work Ben LaHaise did around 2.5.20 time.  I split it
up and updated it for the new preempt_count semantics.

I split the original patch up into 3 pieces (apply in this order):
* clean thread info infrastructure (1/3)
   - take out all instances of things like (8191&addr) to get
     current stack address.
* stack checking (3/3)
   - use gcc's profiling features to check for stack overflows upon
     entry to functions.
   - Warn if the task goes over 4k.
   - Panic if the stack gets within 512 bytes of overflowing.
* interrupt stacks (3/3)
   - allocate per-cpu interrupt stacks.  upon entry to
     common_interrupt, switch to the current cpu's stack.
   - inherit the interrupted task's preempt count

Any suggestions on how to deal with "gcc -p" and old, buggy versions
of gcc would be appreciated.
-- 
Dave Hansen
haveblue@us.ibm.com

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