Set VM_KMEM_SIZE_SCALE to 1 on riscv and arm64

[markj]

Summary

This parameter limits the amount of memory that can be allocated to UMA.
On systems with a direct map and ample KVA, however, I can't see a
reason for VM_KMEM_SIZE_SCALE to be larger than 1. I believe it was
copied from 32-bit ARM to arm64 and from there to riscv.

[alc]

Similarly, there is no reason to define VM_KMEM_SIZE_MIN.

[markj]

Remove VM_KMEM_SIZE_MIN as well.