Encoding conversion with iconv failed.

Please include the following in your bug report:

**Version of emscripten/emsdk:**
v3.1.0

I use "iconv_open("GB18030", "UTF-8"));"

`iconv_t iconv_open(const char *to, const char *from)
{
size_t f, t;
struct stateful_cd *scd;

if ((t = find_charmap(to))==-1
|| (f = find_charmap(from))==-1
|| (charmaps[t] >= 0330)) {
errno = EINVAL;
return (iconv_t)-1;
}`
Why do have this judgment: (charmaps[t] >= 0330), so I cant't convert to GB18030.
Is it not support?

Start with the iconv_open entry point and the reported `(charmaps[t] >= 0330)` check in Emscripten's iconv implementation. Inspect how supported charmaps are defined and how GB18030 is handled; done means establishing whether GB18030 should be supported and resolving or documenting the reported conversion failure.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.