target/i386: Assert CODE64 for x86_64 user-only

target/i386: Assert CODE64 for x86_64 user-only

For x86_64 user-only, there is no way to leave 64-bit mode.

Without x86_64, there is no way to enter 64-bit mode.  There is
an existing macro to aid with that; simply place it in the right
place in the ifdef chain.

Since we're adding an accessor macro, pull the value directly out
of flags when we're not assuming a constant.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210514151342.384376-13-richard.henderson@linaro.org>