target/riscv: Move cpu_get_tb_cpu_state to tcg-cpu.c

This function is only relevant to tcg.
Move it to a tcg-specific file.

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