The default value for CMAKE_INSTALL_PREFIX should be
based on what architecture the built targets are, not
what architecture CMake itself is.

This fix merely guesses better what the built targets
architecture is. It still may guess incorrectly in some
cases. For those cases, it will have to be up to build
scripts and developers on projects to pass in a correct
value for CMAKE_INSTALL_PREFIX with -D on the command line
or via 'force cache value' logic in CMakeLists.txt files.