diexec.c: Cache a pointer directly to the machine scheduler at startup,
to eliminate device().machine().scheduler() chains all over the place (nw)

This change has been sitting in my local tree for months, from when I was
(mostly unsuccessfully) working on c64 performance. It gives a small speedup
to drivers with many tightly-interleaved executing devices (i.e. Commodore
drivers) I didn't think it was worth making everyone do a clean build for
such tiny gains but I didn't have the heart to chuck it. Since I'm already
making another core-touching, clean-build-needed commit tonight, in it goes.