Move RVFI protocol implementation to C

[Timmmm]

After #494 is done we should move the RVFI protocol implementation out of Sail and into C.

[Timmmm]

So I was thinking

1. Move the struct definitions to C structs:

bitfield RVFI_DII_Execution_PacketV2 : bits(512) = {
  magic : 63 .. 0,         // must be set to 'trace-v2'
  trace_size : 127 .. 64,  // total size of the trace packet + extensions
...
}

To this (we can assume little endian):

#pragma pack(push, 1)
struct RVFI_DII_Execution_PacketV2 {
  uint64_t magic;
  uint64_t trace_size;
  ...
};
#pragma pack(pop)

2. Delete rvfi_inst_data, rvfi_pc_data, rvfi_int_data_present, rvfi_mem_data, rvfi_write, rvfi_read, etc.
3. In riscv_callbacks.cpp, wherever it calls zrvfi_something, create a similar function in rvfi_dii.cpp and call that instead.
4. Delete riscv_fetch_rvfi.sail and add a generic mechanism to force instructions into the model. This could be a parameter to try_step() for example. Or a separate zforce_next_instruction() call or something like that.

[arichardson]

I think this is a good idea - CHERI QEMU already has it done in C. Maybe we can reuse some of that.

Some links to the relevant code:
https://github.com/CTSRD-CHERI/qemu/blob/967e7a86d8d7d1b0a730640354269afebaa0c3ed/target/riscv/cpu.c#L428

and https://github.com/CTSRD-CHERI/qemu/blob/967e7a86d8d7d1b0a730640354269afebaa0c3ed/target/riscv/rvfi_dii.h#L43