configure: Don't override ARCH=unknown if enabling TCI

At the moment if configure finds an unknown CPU it will set
ARCH to 'unknown', and then later either bail out or set it
to 'tci' (depending on whether the user passed configure the
--enable-tcg-interpreter switch). This is unnecessarily
confusing, because we could be using TCI in two cases:
 * a known host architecture (in which case ARCH is set to
   the actual host architecture, like 'i386')
 * an unknown host architecture (in which case ARCH is
   set to 'tci')
so nothing can rely on ARCH=tci to mean "using TCI".
Remove the line setting ARCH, so we leave it as "unknown",
which is what the actual situation is.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
