jit: Make calls via cgocall.
gojit/amd64: Port to the new cgo conventions

TODO: CallFunc is going to need some love.
Update Assembler.CallFunc to use the cgo machinery.
Update bf for the new cgo ABI.
clear memory before calling hello

If hello doesn't have clear memory, it's liable to run off the end of
the buffer and corrupt the hell out of memory.
Merge branch 'cgo'
Benchmark an empty JIT call.
Refactor assembler tests, benchmark jit->go calls.