configure: Don't claim 'unsupported host OS' when better message available

The change in commit 898be3e0415c6d which made completely
unrecognized OSes cause an error_exit "Unsupported host OS"
has some unfortunate unintended effects:
 * if you run 'configure --help' on an unsupported host OS
   (eg if intending to use it as a build machine for a
   cross compile to a supported host) then the message
   is printed instead of --help
 * if the C compiler doesn't work or is missing (eg if
   you passed an incorrect --cross-prefix by mistake)
   the message is printed instead of the more useful
   'compiler does not exist or does not work' message

Fix this by postponing the error_exit in this situation
until later, when we have already identified the more
useful cases for this.

The long term fix for this would be to move handling
of --help much further up in the configure script,
and make its output not dependent on checks that configure
runs. However for 2.9 this would be too invasive.

Reported-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Tested-by: Stefan Weil <sw@weilnetz.de>
