Fix synthesis of some configurations

[piotro888]

• After front end refactor icache=False was not synthesizing!!
• Disabled icache and user mode in tiny config
• Added check if FetchResumeKey (from unsafe) is provided. Otherwise tiny config that doesn't have any unsafe instructions would not synthesize
• Add run of asm test tiny core config to prevent that kind of errors issues in future
• Updated tiny core config to implement full I - added ExceptionUnit. Why not? If exception happens it would restart execution to 0x0 (default) instead of hanging up (or worse?) the core in undefined state. CSR Unit is not included so hepefully most of interrupt handling will by optimized away