qemu_v8: do not force COMPILE_NS_USER := 64

32-bit non-secure user space works fine with the QEMUv8 build so set
COMPILE_NS_USER to 64 by default but let the user change this value to
32 if needed ('make COMPILE_NS_USER=32').

Signed-off-by: Jerome Forissier <jerome@forissier.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>