accel/tcg: Avoid reading too much in load_atom_{2,4}

When load_atom_extract_al16_or_al8 is inexpensive, we want to use
it early, in order to avoid the overhead of required_atomicity.
However, we must not read past the end of the page.

If there are more than 8 bytes remaining, then both the "aligned 16"
and "aligned 8" paths align down so that the read has at least
16 bytes remaining on the page.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>