backends: Support returning lvalue and rvalue references

backends: Support returning lvalue and rvalue references

On the low level, they are the same as pointers. The change needs to be
done for all backends, so define a function and a macro to avoid
repetition. Also add a native test, which has to be implemented in C++.
Add the configure check for it.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>