x86: load _nanokernel in %edi in _Swap()

Loading the _nanokernel address in %edi rather than in %eax allows
calling funtions in _Swap() without having to restore it, since %eax is
used for the return value. %edi is a callee-saved register and does not
have to be restored.

Change-Id: I338086d8e15857e835d5d7487de975791926f869
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>