n64-systemtest improvements

[hacktarux]

Add enough fixes to get a simple n64-systemtest with a few arithmetic tests boot in interpreter:

• SC and LL fixes
• naive ADDI without overflow detection
• naive DMTC0 and DMFC0 should be reworked once the registers are not necessarily 32 bits
• make sure permanent loop detection stop the cpu thread once the screen has been updated at least once