encoding/ianaindex: support the GB2312 charset

[larrasket]

The IANA charset GB2312 (MIBenum 2025) resolved to no encoding even
though its aliases were already registered, because the encodings
table had no entry for it.

Map it to GBK, of which GB2312 is a subset, matching x/text's
htmlindex which maps every GB2312 label to GBK. Wrap the encoding so
it reports MIB 2025 and the name GB2312 rather than GBK's own
identifier.

Fixes golang/go#24636

[gopherbot]

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