[cuda] disable threading for now by default
[cuda] disable threading for now by default

There seems to be an atexit() race with the CUDA driver
deinitialization. This causes crashes when accessing a
cuda function from the thread. Can be reproduced more
reliably with POCL_DEBUG=1 as it enables the time stamp
query which crashes usually.

pocl/pocl,franz/pocl,pocl/pocl,Oblomov/pocl,franz/pocl,franz/pocl,Oblomov/pocl,franz/pocl,franz/pocl,pocl/pocl,Oblomov/pocl,pocl/pocl,pocl/pocl,Oblomov/pocl,Oblomov/pocl