configure: Fix test for supported host CPU type

The test for whether the host CPU is supported had several problems:
 * the attempt to fall back to TCI was done as a duplicate
   test, very late (so "--cpu foo" would fail early but "--cpu unicore32"
   would fail late, differently, and after configure had already
   printed a lot of output)
 * a number of CPUs only supported as guests were included in the
   list of CPUs we would accept as valid hosts, which would result
   in a late compile failure on those systems rather than a
   configure failure or fallback to TCI
 * bailing out for an unsupported CPU happened before the main
   option parsing, so "configure --help" wouldn't work

Fix these by folding the setting of ARCH into the first test for
supported host CPU, removing spurious guest-only CPU names from it,
and moving the "fall back to TCI" code earlier.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
