Move `libstdc++` path into `LOADER_*_DEP_LIBS`
After adding `libstdc++` probing into the Julia loader [0], we
originally made the assumption that the `libstdc++` that is shipped with
`julia` would always be co-located with `libjulia.so` [1]. This is not
the case when building with `USE_SYSTEM_CSL=1`, however, where we
sequester system libraries in `usr/lib/julia`, even at build-time.
The path to `libstdc++.so` has already been getting altered when moving
from build-time to install time via `stringreplace` [2], but after
further thought, I decided that it would be better to just use the
pre-existing `LOADER_*_DEP_LIBS` mechanism to communicate to the loader
what the correct relative path to `libstdc++.so` is. This also allows
the single `stringreplace` to update all of our "special" library paths.
[0] https://github.com/JuliaLang/julia/pull/46976
[1] https://github.com/JuliaLang/julia/pull/46976/files#diff-8c5c98f26f3f7aac8905a1074c5bec11a57e9b9c7c556791deac5a3b27cc096fR379
[2] https://github.com/JuliaLang/julia/blob/master/Makefile#L430
