localedata: GBK: add mapping for 0x80->Euro sign [BZ #20864]

Microsoft long ago added a mapping for 0x80 to the Euro sign to their
CP936.  While GBK 1.0 doesn't include this mapping, it is compatible,
and Microsoft and glibc alias the two codepages.  We could split them
apart so GBK wouldn't include the mapping, but that seems like a lot
of work for little gain.