mk: Fix cross-host builds
The change in b20e748 had the unintended consequence of breaking cross-host
builds as we apparently relied on the incorrect definition of this variable in
the makefiles. That change, however, was required to get tests passing so we
couldn't just revert it.
This commit fixes the underlying bug by leaving the "more correct" definition of
`LD_LIBRARY_PATH_ENV_TARGETDIR` (also fixing it with a hardcoded reference to
`CFG_BUILD`) and updating the `RPATH_VAR` definition below. Turned out we
already had special-casing logic for passing `--cfg stage1` during the
well-we-print-this-as-stage0 build of a cross-host. That logic was just updated
to pull from a different variable as opposed to relying on the definition of
that variable to accommodate this.
Closes#32568
b678600ac9