Replace dpkg-architecture by gcc -print-multiarch.

Silent the error on cmake if the flag is not present. Checking the
content of the OUTPUT_VARIABLE for empty should be enough to detect
errors on multiarch detection.
Fix call to get_multiarch by removing the parameter
Improve the method of detecting multiarch

Run a two step approach using gcc -print-multiarch and, if failed,
fallback to use dpkg-architecture.