toolchain: rework C++ options

Instead of having BR2_GCC_CROSS_CXX and BR2_INSTALL_LIBSTDCPP, with
BR2_GCC_CROSS_CXX not being visible (and therefore being useless),
let's just keep BR2_INSTALL_LIBSTDCPP to enable C++ in the toolchain
and install C++ libraries on the target.

We also take that opportunity to make BR2_INSTALL_LIBSTDCPP an hidden
option, which is selected by an option in Buildroot toolchain support
or an option in External toolchain support, just as we did for other
toolchain features.

Some work definitely remains to be done :

 - The name BR2_INSTALL_LIBSTDCPP is ugly, but we keep it for the
   moment in order to avoid changing all packages.

 - We should clarify the other language-related options (Fortran,
   Java, Objective-C, etc.).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>