Split early startup code out of locore.S into locore0.S. Adjust link

Split early startup code out of locore.S into locore0.S.  Adjust link
run so that this locore0.o is always at the start of the executable.
But randomize the link order of all other .o files in the kernel, so
that their exec/rodata/data/bss segments land all over the place.

I haven't worked on the unmap/smashing of the startup code yet.