riscv: Improve virtual kernel memory layout dump

With the arrival of sv48 and its large address space, it would be
cumbersome to statically define the unit size to use to print the different
portions of the virtual memory layout: instead, determine it dynamically.

Signed-off-by: Alexandre Ghiti <alexandre.ghiti@canonical.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>