With the current structure of cputlb.c, there is no difference
between the little-endian and big-endian entry points, aside
from the assert.  Unify the pairs of functions.

Hoist the qemu_{ld,st}_helpers arrays to tcg.c.

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