Apply scounteren only if S-mode is supported (#726)

When U-mode is enabled but S-mode is disabled, `mcounteren` should control the availability of the hardware performance-monitoring counters in U-mode, and `scounteren` should be ignored.
The current implementation (incorrectly) raises an illegal instruction trap when reading performance-monitoring counters in U-mode while S-mode is disabled, even if the counters are enabled in `mcounteren`.