target/riscv: Do not expose rv128 CPU on user mode emulation

target/riscv: Do not expose rv128 CPU on user mode emulation

As Richard mentioned:

  We should allow RV128 in user-mode at all until there's a
  kernel abi for it.

Remove the experimental 'x-rv128' CPU on user emulation
(since it is experimental, no deprecation period is required).

Reported-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>