Move Vivado Mitigations to be ifdef-guarded

[seldridge]

We currently have two Vivado-centric mitigations to address pretty egregious bugs in Vivado tooling. One of these is mitigated with a Vivado-specific SystemVerilog attribute.

Change the SystemVerilog attribute to be ifdef guarded with a macro that is only defined for Vivado. (I don't know what this is, but it's probably XILINX.)

E.g., consider the following FIRRTL circuit:

FIRRTL version 4.2.0
circuit Foo:
  public module Foo:
    input clock: Clock
    input r: {addr: UInt<3>, en: UInt<1>, flip data: UInt<8>}
    input w: {addr: UInt<3>, en: UInt<1>, data: UInt<8>, mask: UInt<1>}

    mem memory:
      data-type => UInt<8>
      depth => 16
      reader => r
      writer => w
      read-latency => 0
      write-latency => 1
      read-under-write => undefined

    connect memory.r.addr, r.addr
    connect memory.r.en, r.en
    connect memory.r.clk, clock
    connect r.data, memory.r.data

    connect memory.w.addr, w.addr
    connect memory.w.en, w.en
    connect memory.w.clk, clock
    connect memory.w.data, w.data
    connect memory.w.mask, w.mask

Change this so that, when compiled it produces:

// Generated by CIRCT firtool-1.108.0-23-g231dea049
// VCS coverage exclude_file
module memory_16x8(
  input  [3:0] R0_addr,
  input        R0_en,
               R0_clk,
  output [7:0] R0_data,
  input  [3:0] W0_addr,
  input        W0_en,
               W0_clk,
  input  [7:0] W0_data
);

  `ifdef XILINX
  (* ram_style = "distributed" *)
  `endif
  reg [7:0] Memory[0:15];
  reg       _R0_en_d0;
  reg [3:0] _R0_addr_d0;
  always @(posedge R0_clk) begin
    _R0_en_d0 <= R0_en;
    _R0_addr_d0 <= R0_addr;
  end // always @(posedge)
  always @(posedge W0_clk) begin
    if (W0_en & 1'h1)
      Memory[W0_addr] <= W0_data;
  end // always @(posedge)
  assign R0_data = _R0_en_d0 ? Memory[_R0_addr_d0] : 8'bx;
endmodule

module Foo(
  input        clock,
  input  [2:0] r_addr,
  input        r_en,
  output [7:0] r_data,
  input  [2:0] w_addr,
  input        w_en,
  input  [7:0] w_data,
  input        w_mask
);

  memory_16x8 memory_ext (
    .R0_addr ({1'h0, r_addr}),
    .R0_en   (r_en),
    .R0_clk  (clock),
    .R0_data (r_data),
    .W0_addr ({1'h0, w_addr}),
    .W0_en   (w_en & w_mask),
    .W0_clk  (clock),
    .W0_data (w_data)
  );
endmodule

As part of this, investigate if there are Vivado-specific macros that it defines, e.g., if XILNIX is actually defined. If no such define exists, then we may need to define one.