cleanup: Add missing feature/ XML files to Makefile

cleanup: Add missing feature/ XML files to Makefile

This patch adds some missing .xml files to features/Makefile so that when the
directory's C files are regenerated, all files are appropriately remade.

This has demonstrated that there have been several "misses" in regenerating
files in this directory. Namely, arm-secext.c and sparc{32,64}-solaris.c. For
the former case, there was what essentially amounts to a typo regarding the
create feature function's name. In the later case, this file has missed at least
one important update in July, 2020, when allocate_target_description was
changed to return a unique pointer.

Those corrections are included.