locore.S: prepare for C function calls earlier

[mhorne]

Summary

First, set the stack pointer right away, and zero the frame pointer.

Second, switch the boot argument registers to the unused s3 and s4. This
ensures the values will not be clobbered by function calls; they are
consumed late in the assembly routine.

[jrtc27]

As with D47458, C code should not be run prior to enabling translation so we're running at the right address, so IMO this patch is a sign of going down the wrong road. Yes, Linux does horrendously fragile things like this, but that doesn't mean we should do the same.