v0.40.0

The compiler for the other machine

The front end had a second machine, so the chain closed here and
cross-compiled for there. The compiler itself was still for one, which was
the last place a toolchain could have been hiding.

Now it is the same compiler with the other tail, and it settles over there
the way the first settles here: the seed writes one that emits for that
machine, it builds itself over there, and the two generations after it are
the same octets. What the front end it builds writes is this file's own
output for that machine, octet for octet.

One difference is forced. A load on that machine carries an offset that is
unsigned and scaled, so a frame cannot be addressed downwards from its top; a
register points at the floor and every slot is a positive offset up. The
busiest function in any of these programs wants six hundred and seventy-one
values at once, which is well inside what that reaches, and the check that
says so fails while failing is cheap.

The whole thing is asked of both machines now.

Full Changelog: v0.39.0...v0.40.0

LOC: 21965