Previously, the RAM is synchronous.
However, due to verilator issues, the bug is hidden by Buffer that includes FFs.

The buffer works as follows (simplified):
always @(posedge clk)
  data_out <= data_form_ram_helper;

data_from_ram_helper is given by (synchronous read):
always @(posedge clk)
  ram_helper(raddr, data_from_ram_helper);

At every positive edge, data_out should be assigned with data_from_ram_helper_old,
and data_from_ram_helper should then be evaluated to the new value during the next clock cycle.
However, verilator evaluates data_from_ram_helper first and then assigns it to data_out,
that is, data_out is incorrectly assigned by data_from_ram_helper.

For example, verilator gives the following sequence:
 raddr    data_from_ram_helper     data_out
  0               X                    X
  1             data[0]             data[0]
  2             data[1]             data[1]

However, the correct values should be:
 raddr    data_from_ram_helper     data_out
  0               X                    X
  1             data[0]                X
  2             data[1]             data[0]

Previously, due to the two bugs, ram works well.
However, when it comes to multi-threading, they are put to two threads
and since verilator does not find the relationship between raddr and data_from_ram_helper, data_out,
they don't follow any specific evaluation order.
Thus, multi-threaded emu randomly produces difftest error.

To prove that verilator incorrectly evaluates DPI-C functions and related signals
(however, it's also possible that we were using DPI-C functions incorrectly),
one can change ram.v to
  always @(posedge clk) begin
    rdata <= ram_read_helper(raddr);
    ram_write_helper(waddr, wdata);
  end
This should be the same with previous version of ram.v but it will give errors on difftest.

To solve the issue, this commit makes two modifications:
(1) make the ram asynchronous
AXIWrapper requests the RAM to be asynchronous such that after ar.fire() we have eight cycles of rdata[0-7].
(2) changes DPI-C function prototype to uint64_t ram_read_helper(uint64_t raddr)
In this form, verilator detects the correct order between data_from_ram_helper and data_out evaluation.

* by default, snapshot will be saved every 10s

* now mainargs is supported by nexus-am

* To run more programs on debian, we use 256MB memory described in dts.
  But if the hardware still provides 128MB memory, the memory access
  will wrap around when Linux allocate a physical page excceed 128MB,
  which causes wrong memory access.

* also put mainargs at 0x80001000 to match AM

* now we can pass the image used in AXI4RAM as an argument of emu