target/rx: update PC correctly in wait instruction

`cpu_pc` at this point does not necessary point to the current
instruction (i.e., the wait instruction being translated), so it's
incorrect to calculate the new value of `cpu_pc` based on this. It must
be updated with `ctx->base.pc_next`, which contains the correct address
of the next instruction.

This change fixes the wait instruction skipping the subsequent branch
when used in an idle loop like this:

    0:  wait
        bra.b 0b
        brk   // should be unreachable

Signed-off-by: Tomoaki Kawada <i@yvt.jp>
Reviewed-by: Yoshinori Sato <ysato@users.sourceforge.jp>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220417060224.2131788-1-i@yvt.jp>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>