add libstdc++ subpackage to libgcc

[bcressey]

Issue number:
Related: bottlerocket-os/bottlerocket-sdk#215

Description of changes:
Add a libstdc++ subpackage to the libgcc package, so that downstream packages that use C++ can arrange for the library to be installed at runtime without including it in their own files.

Testing done:
Extracted the packaged files and confirmed that the library and symlink were correct:

❯ ls -latr ./*-bottlerocket-linux-gnu/sys-root/usr/lib/
./aarch64-bottlerocket-linux-gnu/sys-root/usr/lib/:
total 2176
lrwxrwxrwx. 1 fedora fedora      19 Nov 11 19:59 libstdc++.so.6 -> libstdc++.so.6.0.29
-rwxr-xr-x. 1 fedora fedora 2216376 Nov 11 19:59 libstdc++.so.6.0.29

./x86_64-bottlerocket-linux-gnu/sys-root/usr/lib/:
total 2212
lrwxrwxrwx. 1 fedora fedora      19 Nov 11 18:53 libstdc++.so.6 -> libstdc++.so.6.0.29
-rwxr-xr-x. 1 fedora fedora 2256504 Nov 11 18:53 libstdc++.so.6.0.29

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.