verilog_backend: in non-SV mode, add a trigger for `always @*`.

verilog_backend: in non-SV mode, add a trigger for `always @*`.

This commit only affects translation of RTLIL processes (for which
there is limited support).

Due to the event-driven nature of Verilog, processes like

    reg x;
    always @*
        x <= 1;

may never execute. This can be fixed in SystemVerilog code by using
`always_comb` instead of `always @*`, but in Verilog-2001 the options
are limited. This commit implements the following workaround:

    reg init = 0;
    reg x;
    always @* begin
        if (init) begin end
        x <= 1;
    end

Fixes #2271.