tools: ynl-gen: generate enum-to-string helpers

It's sometimes useful to print the name of an enum value,
flag or name of the op. Python can do it, add C helper
code gen for getting names of things.

Example:

  static const char * const netdev_xdp_act_strmap[] = {
	[0] = "basic",
	[1] = "redirect",
	[2] = "ndo-xmit",
	[3] = "xsk-zerocopy",
	[4] = "hw-offload",
	[5] = "rx-sg",
	[6] = "ndo-xmit-sg",
  };

  const char *netdev_xdp_act_str(enum netdev_xdp_act value)
  {
	value = ffs(value) - 1;
	if (value < 0 || value >= (int)MNL_ARRAY_SIZE(netdev_xdp_act_strmap))
		return NULL;
	return netdev_xdp_act_strmap[value];
  }

Signed-off-by: Jakub Kicinski <kuba@kernel.org>