updated the dev packaging for the hhvm libdir and gnuinstalldirs

The dpkg-architecture command is used to determine the architecture for building packages on debian based systems. According to the man page the DEB_BUILD_GNU_TYPE variable is "The GNU system type of the build machine." I would use the DEB_BUILD_MULTIARCH variable which is: "The  clarified  GNU  system  type  of  the  build machine, used for filesystem paths (since dpkg 1.16.0)" but debian 10 does not have dpkg 1.16. It appears the values of those variables are the same however. Also, on some systems you will need to install dpkg-dev for the dpkg-architecture command since it is not distributed on older releases.