RISC-V: Zicbo{m,p,z} adjustments to riscv_multi_subset_supports_ext()

The lack thereof did caused gas to issue "internal: unreachable
INSN_CLASS_*" errors when trying to assemble respective insns without
the feature(s) enabled via e.g. ".option arch, ...". Of course a proper
hint towards the missing extension then wasn't given either.
1 file changed