RISC-V: Ensure kernel stack is aligned

[jrtc27]

Summary

Currently, PCB_SIZE is an odd number of 8 bytes, as is 40, the size of
riscv_bootparams, so we happen to have an aligned kernel stack. Instead
we should round the allocation up as required. We can also fold the two
together to avoid unnecessarily padding each.

[br]

Looks good. Thank you!