host architecture detection binary not found

After commit 89aa6e78 (Fix determination of host_arch test binary), canadian cross-compilation fails to find the host architecture detection binary.

In Yocto, all Qt builds are always single architecture builds. Mingw builds are done with Qt configured as "-platform win32 -xplatform win32 -external-hostbindir ..." In these cases, configure tries to find 'arch_host'  binary without the windows .exe postfix, eventhough the test is build as a windows binary.

Can be reproduced as desktop build on linux machine with modified win32-gcc mkspec, by replacing the ${CROSS_COMPILE} with the actual prefix.