Don't call time_update() when debug stats are off.

This is not fast especially when syscalls are being hammered.
Optimize sysclock / sysclock 2 usec.

Legend of Heroes 1/2/3 call these a ton, making them quite slow.
Not sure if it's a bug, but they can be more optimal which'll make
it easier to debug anyway.
Merge pull request #575 from unknownbrackets/perf

Improve performance of sceKernelSysClock2USec