We expect the backend to require register pairs in
host-endian ordering, thus for big-endian the first
register of a pair contains the high part.
We were forcing R0 to contain the low part for calls.

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