[rfc] stack alignment macro cleanup

[Paul Brook]

> I would like to reduce this to
> 
> STACK_BOUNDARY
> 
>   -- minimum alignment enforced by hardware.
>...
>   -- unchanged

This may be determined by factors other than hardware.  For example the ARM 
EABI requires that the stack be 8-byte aligned at public entry points. However 
within a function the stack pointer is only required to be 4-byte aligned.

We currently set S_B=64 and hope nothing notices that the prologue/epilogue 
involve transient 32-bit aligned values.

Your proposal doesn't make this problem any worse, if anything it's better 
because we don't have to device between S_B and PREFERRED_STACK_BOUNDARY. I'm 
just noting that documenting this as a hardware property is at best 
misleading.

Paul