It's been a while since RISC-V support was added to CT-NG in 2017.
Since then RISC-V support was integrated in all the key toolchain
components upstream and now are proven to be in a very good state.
Thus it makes no sense to keep this architecture "hidden" in
experimental options, so we promote RISC-V architecture in CT-NG.
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>