[Sbcl-devel] make-target-2-load.lisp: add :ucrt to public features

[Masatoshi S. <sn...@gm...>]

:ucrt is detected at build time (via tools-for-build/ucrt-test.c) and used
as a reader conditional in several places, but it is missing from the
public-features whitelist in make-target-2-load.lisp and therefore stripped
from *features* before the core is saved.

This means the feature does not work as intended when it matters most:
contribs are built using the saved core, so #+ucrt/#-ucrt conditions in
contrib code (currently sb-posix/constants.lisp and
sb-posix/interface.lisp) are never taken.