configure: Don't allow user-only targets for unknown CPU architectures

configure: Don't allow user-only targets for unknown CPU architectures

For the user-only targets, we need to know something about the host CPU
architecture even if we are using the TCI interpreter rather than TCG.
(In particular user-exec.c has code for handling signals that needs
to know about that host's context structures.)

Specifically forbid building the user-only targets on unknown CPU
architectures, rather than allowing them to configure but then fail
when building user-exec.c.

This change drops supports for two configurations which were theoretically
possible before:
 * linux-user targets on M68K hosts using TCI
 * linux-user targets on HPPA hosts using TCI

We don't think anybody is actually trying to use these in practice, though:
 * interpreted TCG on a slow host CPU would be unusably slow
 * the m68k user-exec.c support is missing is_write detection so guest
   code which writes to the same page it is executing from was broken
   (will include any guest program using signals)
 * HPPA TCG backend support was dropped two and a half years ago
   with no complaints

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>