CMakeFindPackageMode: Fix Debian multiarch default guess (#15250)

In commit v2.8.6~102^2~18 (Better support for lib64 and Debian
multiarch, 2011-07-14) variable names "..._LANGUAGE_ARCHITECTURE" were
set instead of the correct "..._LIBRARY_ARCHITECTURE".  Fix this typo
and also set the language-independent variable
CMAKE_LIBRARY_ARCHITECTURE.