ci: enable C++ check for RISC-V

ci: enable C++ check for RISC-V

The current packages installed for RISC-V build check do not contain a
C++ compiler, which hid an issue with C++ type conversion in the
rte_vect.h header on RISC-V or in the scalar implementation of the LPM
x4 lookup. Now that this issue is fixed, use the full toolchain install
to enable the C++ test.

Besides, the user's guide for RISC-V cross-compilation recommends the
use of crossbuild-essential-riscv64.

Signed-off-by: Stanislaw Kardach <kda@semihalf.com>
Reviewed-by: David Marchand <david.marchand@redhat.com>
Acked-by: Aaron Conole <aconole@redhat.com>