disable qthreads internal spinlocks for pgi/PrgEnv-pgi

pgi doesn't support the proper atomic intrinsics that are required for
spinlocks to be fast, so disable them for pgi, and PrgEnv-pgi
$(findstring pgi,$(CHPL_MAKE_TARGET_COMPILER)) would probably be more robust against future gen compiler name changes than $(filter ...) will be, and personally I think it's easier to read for this sort of test. I'll grant the latter is a matter of taste, though, and it's okay with me if you commit it as is.