gcc: move C++ support option next to Fortran/Objective-C

The option to enable C++ support was still located in
toolchain/toolchain-buildroot/Config.in.2, with misc other toolchain
options. It seems more logical to have this option with the other
options to select the languages supported by the cross-compiler, so we
move it next to the Fortran/Objective-C options in
package/gcc/Config.in.host.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>