[riscv] Add support for enabling 32-bit paging

[riscv] Add support for enabling 32-bit paging

Add code to construct a 32-bit page table to map the whole of the
32-bit address space with a fixed offset selected to map iPXE itself
at its link-time address, and to return with paging enabled and the
program counter updated to a virtual address.

Signed-off-by: Michael Brown <mcb30@ipxe.org>