accel/tcg: Use DisasContextBase in plugin_gen_tb_start

Use the pc coming from db->pc_first rather than the TB.

Use the cached host_addr rather than re-computing for the
first page.  We still need a separate lookup for the second
page because it won't be computed for DisasContextBase until
the translator actually performs a read from the page.

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