Introduce 'noimpl' attribute for natives

This replaces the old method - setting the value of the 'native'
attribute to 'special' for those natives that are implemented manually.
Fix codegen not making directory for output file
Revert "Remove cmake/BuildConfig.cmake"

This reverts commit bc0f25ad7df344704fa46c31d0f2eaa66261276b.
Revert "Add -m32 to compile flags"

This reverts commit 12515fd9e27c97a3edb150e8dc2fbd28581a3b55.
Replace ugly IS_ABSOLUTE checks with get_filename_component()
Correct source file order