accel/tcg: Introduce tb_pc and log_pc

The availability of tb->pc will shortly be conditional.
Introduce accessor functions to minimize ifdefs.

Pass around a known pc to places like tcg_gen_code,
where the caller must already have the value.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>