Remove savedIdRegister/MIPS_CALL_ID.

We've never trusted it anyway, simpler without dealing with this stuff.
Improve sceKernelDelayThread() timing.

A delay of 0 never reschedules, actually, unless there's a better priority
thread.  A greater delay depends on if a >= or similar check matches, but
generally is at least 200us.

It seems like this actually completes in at least another 10-20us.
Eat some cycles when switching threads.

There's a cost and it seems to be at least this much.  Matters for
Wipeout Pure when callbacks are allowed to wait.
Fix a warning on 32-bit that might be bad...
Merge pull request #6403 from unknownbrackets/sched-cycles

Eat cycles when context switching and delaying