configure: Don't override user's --cpu on MacOS and Solaris

Both MacOS and Solaris have special case handling for the CPU
type, because the check_define probes will return i386 even if
the hardware is 64 bit and x86_64 would be preferable. Move
these checks earlier in the configure probing so that we can
do them only if the user didn't specify a CPU with --cpu. This
fixes a bug where the user's command line argument was being
ignored.

Reviewed-by: Andreas F=E4rber <afaerber@suse.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
