sim: cgen: tweak initializers to avoid warnings

Use {} instead of {0} to avoid warnings:

common/cgen-utils.c:59:1: warning: missing braces around initializer [-Wmissing-braces]
   59 | {
      | ^
   60 |   {
   61 |     VIRTUAL_INSN_X_INVALID, "--invalid--", NULL, 0, { V, { 0 } }
      |                                                            {{}}

Generated code should be the same.