target/riscv: Replace `tb_pc()` with `tb->pc`

Signed-off-by: Anton Johansson <anjo@rev.ng>
Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230227135202.9710-18-anjo@rev.ng>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>