Consolidate and add files to clean target variables

Consolidate and add files to clean target variables

To increase the consistency of how automatic clean targets run,
define the variables together without +=, default to MOSTLYCLEANFILES
when there is no need for different levels or
add more clean levels to match other subdirectories,
add more files that are built, remove duplication, and cleanup.

Do the same for EXTRA_DIST where it is equally messy.

    * backends/Makefile.am: add more objects to clean, improve spacing.
    * debuginfod/Makefile.am: Likewise, and remove duplicates.
    * lib/Makefile.am: improve spacing.
    * libasm/Makefile.am: add more objects to clean, split similar to debuginfod.
    * libcpu/Makefile.am: use normal =, add more objects to clean.
    * libdw/Makefile.am: add more objects to clean, split similar to debuginfod.
    * libdwelf/Makefile.am: add more objects to clean, use lowest clean level.
    * libdwfl/Makefile.am: Likewise.
    * libebl/Makefile.am: add more objects to clean.
    * libelf/Makefile.am: add more objects to clean, split similar to debuginfod.
    * src/Makefile.am: consolidate including EXTRA_DIST, split clean levels,
      define with normal =, define with variables.
    * tests/Makefile.am: Likewise, but not including EXTRA_DIST.

Signed-off-by: Michael Pratt <mcpratt@pm.me>