Let automake manage whether the objects are included in lib.a. This
fixes failures after to commit 71086e8b2d70c1e71a8372f35d9901505fc72905
("newlib: delete (most) redundant lib_a_CCASFLAGS=$(AM_CCASFLAGS)") due
to automake generating different set of implicit rules, and the code in
here assuming the names of the generated objects.
4574c60378