Tweak the context data a bit and do some code cleanup. Save %edx as well

Tweak the context data a bit and do some code cleanup.  Save %edx as well
as %eax and %ecx so we can throw the upcall pointer into %edx, which makes
it possible to create self-contained context assembly code.  This also
improves performance by giving the context assembly (see
src/test/sysperf/upcall.S) easier access to the upcall structure.

It's also safer (though not critical) to have the kernel save all the
call-used registers when setting up for an upcall.