target/arm: Fix test of TCG_OVERSIZED_GUEST

The symbol is always defined, even if to 0.  We wanted to test for
TCG_OVERSIZED_GUEST == 0.

This fixed, the #error is reached while building arm-softmmu, because
TCG_OVERSIZED_GUEST is not true (nor supposed to be true) for arm32
guest on a 32-bit host.  But that's ok, because this feature doesn't
apply to arm32.  Add an #ifdef for TARGET_AARCH64.

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