spinlock_test: allow command-line setting of threads and iterations.
spinlock_test: allow command-line setting of threads and iterations.

mcanthony/oiio,bdeluca/oiio,OpenImageIO/oiio,micler/oiio,cwilling/oiio,OpenImageIO/oiio,YangYangTL/oiio,sambler/oiio,bdeluca/oiio,scott-wilson/oiio,scott-wilson/oiio,lgritz/oiio,YangYangTL/oiio,micler/oiio,lgritz/oiio,YangYangTL/oiio,lgritz/oiio,sambler/oiio,OpenImageIO/oiio,bdeluca/oiio,cwilling/oiio,YangYangTL/oiio,bdeluca/oiio,lgritz/oiio,sambler/oiio,sambler/oiio,mcanthony/oiio,micler/oiio,scott-wilson/oiio,OpenImageIO/oiio,mcanthony/oiio,scott-wilson/oiio,cwilling/oiio,micler/oiio,mcanthony/oiio,cwilling/oiio