*: Remove __i386__ tests

Remove instances of __i386__, except from tests and imported headers.

Drop a block containing sanity check and fprintf error message for
i386-on-i386 or x86_64-on-x86_64 emulation.  If we really want
something like this, we would do it via some form of compile-time check.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>