locore.S: stash boot arguments in saved registers

[mhorne]

Summary

Switch the boot argument registers to the unused s3 and s4. This ensures
the values will not be clobbered by SBI or 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.

[mhorne]

Reduce the change, just switch to using s3/s4 registers, still needed to
preserve the values across SBI calls.