cleanup utf-8 multibyte code, use visibility if possible

cleanup utf-8 multibyte code, use visibility if possible

this code was written independently of musl, with support for a the
backwards, nonstandard "31-bit unicode" some libraries/apps might
want. unfortunately the extra code (inside #ifdef) makes the source
harder to read and makes code that should be simple look complex, so
i'm removing it. anyone who wants to use the old code can find it in
the history or from elsewhere.

also, change the visibility of the __fsmu8 state machine table to
hidden, if supported. this should improve performance slightly in
shared-library builds.