Previously, stdenv used `adjacentPackages==null` to signal that
`actuallySplice` should be `false`.  Let's pass it explicitly for
greater clarity.

It's quite difficult to understand what this `selfBuild` parameter
is signaling, but it turns out to simply indicate which stages have
any kind of cross compilation (i.e. either build!=host or
host!=target).

Let's check that condition directly for greater clarity.

The native stdenv bootstrap (e.g. `pkgs/stdenv/linux/default.nix`)
is very elegantly structured as a sequence of arbitrarily-many
stages.  This makes it easy to refactor and modularize and reuse,
because if a stage gets too complicated we can just split it up into
two separate sequential stages.  We can also insert optional stages.

Unfortunately we can't do the same thing with cross-compiled
stdenvs, because `adjacentPackages` includes the hardwired
assumption that there are exactly two stages after the end of the
native bootstrap stage sequence:

  - The (build==host)!=target stage
  - The build!=(host==target) stage

This commit eliminates that assumption.  Instead of resolving things
like `pkgsBuildHost` to `prevStage`, we instead *search backwards*
for the most recent stage which creates packages on our
buildPlatform and for our hostPlatform.

This commit should not affect eval.

Note that `pkgsTargetTarget`, which becomes `targetPackages` still
has hardwired assumptions.  I think setting `pkgsTargetTarget` to
`nextStage` is just flat-out wrong for every stage except the "main"
stage (the `build!=(host==target)` stage), but I fear that there may
be code that relies on the particular way that it is wrong.  It
looks like `pkgsTargetTarget` and `targetPackages` exist exclusively
in order to provide access to the `postStage` hack; I intend to
submit a separate PR that `throw`s if it is used in any other way,
to see what breaks.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

This commit inserts extra stages into the cross-compilation stdenv
bootstrap in order to prevent assumptions about the number of stages
from creeping back in (see previous commit).

If this commit is used without the previous commit, eval will fail
badly.  The no-op stages added by this commit serve as an example of
what wasn't possible before the PR which includes them.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

This commit inlines the `actuallySplice` variable, which is simply
`stdenv.buildPlatform!=stdenv.targetPlatform`.

Since it isn't anything mysterious, the code becomes much easier to
understand when written that way.

This commit also restructures `stdenvBootstappingAndPlatforms`, but
in a way that makes absolutely no change to its behavior.