Remove flagsToBC boot routine.

Makes boot binary a bit bigger, but that "_pdacc" word will be
reused in high level apps.

Only a few bytes saved in forth1.bin, but the DO .. LOOP construct
isn't used much yet. It's still significant savings per LOOP call.

Allow us to rely a little less on labels. We now only need 4.

It's not used

It makes the tool much more versatile. I'll have adev being included
in all recipes, so it can be assumed.

The former was only used in the peculiar context of "/emul". The
regular case is actually HERE pointing to RAMEND on boot.

This allows optional subsystems to not needlessly reserve too much
system memory and it also reduces offsets bookkeeping.

so that it isn't picked up automatically at boot. Only when (c<$) is
called do we override (c<). This fixes tricky boot problems.

Let's revisit the relinking approach, with LITA this time.
This reverts commit 6652125d47.

It seems that xcomp can't manage to reliably compile core+ units.
Let's limit the role of xcomp to z80c+icore.

I'm getting a little desperate here. I'll revisit the link.fs approach
with a new idea I had...

Replace its usages with direct RAM+ offsets. The (sysv) mechanism was
incompatible with cross-compilation of a full interpreter.

It's not needed anymore now that the boot binary is built in one pass.

... instead of building boot.fs, and then z80c.fs/icore.fs in separate
passes. This strengten xcomp by making it support 2 completely separate
dicts (that is, the X dict is *not* connected to the system dict. The chain
now stops at EXIT def in boot.fs).

We can now cross-compile core.fs.

It was a dead end. my new tentative solution is xcomp.