.github: Add eclass/usr-ldscript.eclass to automation

It couldn't be added before to automation, because the eclass in
Gentoo would introduce have unwanted side-effects into our built
images. But the Gentoo version of the eclass is essentially a no-op
when USE=split-usr is disabled. We have recently moved to use Gentoo
profiles that disable USE=split-usr altogether, and with this move, we
can now safely put the eclass into automation.

This marks the point where the entirety of the portage-stable is under
automation.

.github/workflows/portage-stable-packages-list

@@ -461,9 +461,7 @@ eclass/tree-sitter-grammar.eclass
 eclass/udev.eclass
 eclass/unpacker.eclass
 eclass/user-info.eclass
-# This file is modified by us to be an empty file, so can't be synced for now.
-#
-# eclass/usr-ldscript.eclass
+eclass/usr-ldscript.eclass
 eclass/vcs-clean.eclass
 eclass/vcs-snapshot.eclass
 eclass/verify-sig.eclass