tcg: Allocate TCGTemp pairs in host memory order

tcg: Allocate TCGTemp pairs in host memory order

Allocate the first of a pair at the lower address, and the
second of a pair at the higher address.  This will make it
easier to find the beginning of the larger memory block.

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