Preliminary support for MMU emulation

To boot a 32-bit RISC-V Linux with MMU, we must first support MMU
emulation. The virtual memory scheme to be supported is Sv32. The major
changes in this commit include implementing the MMU-related riscv_io_t
interface and binding it during RISC-V instance initialization. To
reuse the riscv_io_t interface, we modified its prototype.

For each memory access, the page table is walked to get the
corresponding PTE. Depending on the PTE retrieval, several page faults
may need handling. Thus, three exception handlers have been introduced:
insn_pgfault, load_pgfault, and store_pgfault, used in MMU_CHECK_FAULT.
This commit does not fully handle access faults since they are related
to PMA and PMP, which may not be necessary for booting 32-bit RISC-V
Linux.

Since Linux has not been booted yet, a test suite is needed to test the
MMU emulation. This commit includes a test suite that implements a
simple kernel space supervisor and a user space application. The
supervisor prepares the page table and then passes control to the user
space application to test the aforementioned page faults.

Some S-mode CSRs are added to riscv_internal to support S-mode. PTE,
S-mode, and M-mode CSR helper macros are also introduced.

Related: sysprog21#310

src/decode.c

@@ -907,9 +907,8 @@ static inline bool op_system(rv_insn_t *ir, const uint32_t insn)
     default: /* illegal instruction */
         return false;
     }
-    if (!csr_is_writable(ir->imm) && ir->rs1 != rv_reg_zero)
-        return false;
-    return true;
+
+    return csr_is_writable(ir->imm) || (ir->rs1 == rv_reg_zero);
 }
 
 /* MISC-MEM: I-type