Fix self-contained handling of ASCII encoding

D898 was primarily intended to fix hangs in the event that iconv was
unavailable (namely #10298 and #7695). In addition to this fix, it also
introduced self-contained handling of ANSI terminals to allow compiled
executables to run in minimal environments lacking iconv.

However, the behavior that the patch introduced is highly suspicious.
Specifically, it gives the user a UTF-8 encoding even if they requested
ASCII.

This has the potential to break quite a lot of code. At very least it
breaks GHC's Unicode terminal detection logic, which attempts to catch
an invalid character when encoding a pair of smart-quotes. Of course,
this exception will never be thrown if a UTF-8 encoder is used.

Here we use the `char8` encoding to handle requests for ASCII encodings.

Fixes #10623.