Add OBJECT_FILE_DIR rule placeholder for compilation lines

Some compilers do not offer an option to specify the path to the object
file, but rather only to the directory in which to place the object
file.  See issue 14876 for some examples.  Add a new OBJECT_FILE_DIR
placeholder to specify the directory containing the object file for the
current compilation.  This may differ from the main target OBJECT_DIR
when the object corresponds to a source in a subdirectory.
Merge topic 'compile-OBJECT_FILE_DIR' into next

8256ccb7 Add OBJECT_FILE_DIR rule placeholder for compilation lines