x86: properly align initial dummy thread

x86-32 thread objects require special alignment since they
contain a buffer that is passed to fxsave/fxrstor instructions.
This fell over if the dummy thread is created in a stack frame.

Implement a custom swap to main for x86 which still uses a
dummy thread, but in an unused part of the interrupt stack
with proper alignment.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>