Code review fixes

1. Fix all indentation/braces issues
2. Replace POCL_LEAVE_TEMP_DIRS with POCL_LEAVE_KERNEL_COMPILER_TEMP_FILES and retain all intermediate files in-case of latter
3. Place kernel cache in ~/.pocl/kcache/ now
4. Force clear cache when POCL_CLEAR_KERNEL_CACHE is set
5. Enclose pocl_check_and_invalidate_cache() with mutex lock
6. Capture timestamp of every build. Evict kernel-cache when build timestamp changes