target/riscv: Add checks for several RVC reserved operands

C.ADDI16SP, C.LWSP, C.JR, C.ADDIW, C.LDSP all have reserved
operands that were not diagnosed.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>