lib: cpp: Rename CONFIG_RTTI to CONFIG_CPP_RTTI

The Kconfig `CONFIG_RTTI` symbol enables the C++ Run-Time Type
Information (RTTI) support and is a feature of the standard C++
library; therefore, it should be prefixed with `CPP_` as with the rest
of the C++-specific symbols.

This commit renames the Kconfig `CONFIG_RTTI` symbol to
`CONFIG_CPP_RTTI` in order to better align with the other existing C++
Kconfig symbols, and deprecates the old `CONFIG_RTTI` symbol.

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>