accel/tcg: Fix jump cache set in cpu_exec_loop

Assign pc and use store_release to assign tb.

Fixes: 2dd5b7a1 ("accel/tcg: Move jmp-cache `CF_PCREL` checks to caller")
Reported-by: Weiwei Li <liweiwei@iscas.ac.cn>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>