target/arm: Split contiguous stores for endianness

We can choose the endianness at translation time, rather than
re-computing it at execution time.

Tested-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20181005175350.30752-12-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>