accel/tcg: Do not align tb->page_addr[0]

Let tb->page_addr[0] contain the address of the first byte of the
translated block, rather than the address of the page containing the
start of the translated block.  We need to recover this value anyway
at various points, and it is easier to discard a page offset when it
is not needed, which happens naturally via the existing find_page shift.

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