Implement address translation in emu/riscv-mmu.h

[michaeljclark]

Implement instruction fetch, loads and stores using current privilege level and flags e.g. MSTATUS.{MPRV,PUM,MXR,VM}

• fetch_inst
• load
• store
• walk_page_table

Implement a minimal subset of PMA with PMP attributes (emu/riscv-pma.h) to protect M-mode PA from S-mode.

switch (proc.mstatus.vm) {
    case riscv_vm_mbare: /* TODO */ break;
    case riscv_vm_mbb:   /* TODO */ break;
    case riscv_vm_mbid:  /* TODO */ break;
    case riscv_vm_sv32:  /* TODO */ break;
    case riscv_vm_sv39:  /* TODO */ break;
    case riscv_vm_sv48:  /* TODO */ break;
}

[michaeljclark]

Address translation as per privileged spec 1.9.1 has been implemented