When we had configure scripts in subdirs, the newlib_basedir value
was computed relative to that, and it'd be the same when used in the
Makefile in the same dir. With many subdir configure scripts removed,
the top-level configure & Makefile can't use the same relative path.
So switch the subdir Makefiles over to abs_newlib_basedir when they
use -I to find source headers.
Do this for all subdirs, even ones with configure scripts and where
newlib_basedir works. This makes the code consistent, and avoids
surprises if the configure script is ever removed in the future as
part of merging to the higher level.
Some of the subdirs were using -I$(newlib_basedir)/../newlib/ for
some reason. Collapse those too since newlib_basedir points to the
newlib source tree already.
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