cpu-all.h: Remove unnecessary target-specific ifdef for CPU_QuadU

CPU_QuadU isn't used on all targets, but there's no harm in defining the
typedef anyway. It only needs to be guarded by CONFIG_SOFTFLOAT, because
softfloat-native doesn't have a float128 type. This avoids the need for
every new target which uses CPU_QuadU to add itself to an #ifdef in
what ought to be target-agnostic code.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>