qemu_v8: do not force COMPILE_S_USER := 64

32-bit TAs work fine with the QEMUv8 build so set COMPILE_S_USER to 64
by default but let the user change this value to 32 if needed ('make
COMPILE_S_USER=32').

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