JIT: implement timer support in mtspr

Faster, of course, since we avoid the interpreter, but also means we can
get more a more accurate timer in long blocks by adding the offset from the
start of the block to the retrieved timer. I don't know if this will actually
fix any issues, but it's more correct and a nearly-free improvement.
JIT: merge paired timebase reads where possible

Combined with the previous patch, ~1% faster overall on F-Zero GX.
Merge pull request #840 from FioraAeterna/jittimer

JIT: implement timer support in mtspr