Improve perf of sceKernelCheckCallback().

Some games seem to call this a lot.
Improve perf of __KernelExecutePendingMipsCalls().

This was using ~3% in many games with fast forward.  It was all
the RTL type lookup, so avoiding it is an easy win.
Merge pull request #925 from unknownbrackets/perf

Improve perf for thread switching / callbacks