Now works with cross and old and new meta checks alike.

The old way depended on old list-of-strings `meta.platforms`, and was
not good for cross.

Negative reasoning like `allBut` is a bad idea with an open world of
platforms. Concretely, if we add a new, quite different sort of
platform, existing packages with `allBut` will claim they work on it
even though they probably won't.

… world

First, we need check against the host platform, not the build platform.
That's simple enough.

Second, we move away from exahustive finite case analysis (i.e.
exhaustively listing all platforms the package builds on). That only
work in a closed-world setting, where we know all platforms we might
build one. But with cross compilation, we may be building for arbitrary
platforms, So we need fancier filters. This is the closed world to open
world change.

The solution is instead of having a list of systems (strings in the form
"foo-bar"), we have a list of of systems or "patterns", i.e. attributes
that partially match the output of the parsers in `lib.systems.parse`.
The "check meta" logic treats the systems strings as an exact whitelist
just as before, but treats the patterns as a fuzzy whitelist,
intersecting the actual `hostPlatform` with the pattern and then
checking for equality. (This is done using `matchAttrs`).

The default convenience lists for `meta.platforms` are now changed to be
lists of patterns (usually a single pattern) in
`lib/systems/for-meta.nix` for maximum flexibility under this new
system.

Fixes NixOS#30902

`packagePlatforms` now filters `supportedSystems` with the new-style
`meta.platforms`, rather than just plopping it in as is.

I need it in stdenv and release-lib, so that seems motivation enough.

…erns

Instead of intersecting system strings, we filter with the sort of
patterns used in `meta.platforms`.

Indicating this change `forTheseSystems` has been renamed to
`forMatchingSystems`, since the given list is now patterns to match, and
not the systems themselves. [Just as with `meta.platforms`, systems
strings are also supported for backwards compatibility.]

This is more flexible, and makes the `forMatchingSystems` and
packagePlatforms` cases more analogous.

Github broke oddly on my previous PR, so I tested and merged by hand.
Otherwise ofborg would have caught this.

Should have commited on here and on merged master to begin with, but I
didn't, so instead I cherry-pick.

(cherry picked from commit 88c04a8)

Otherwise obscure cross-compilations are hampered. `all` breaks all but
the initial derivation (which we can't even write yet) in an open world
setting however, so we really shouldn't have it.

(cherry picked from commit 3c8ae01)

(cherry picked from commit d9a1800)

(cherry picked from commit e49c14c)

Since GHC is a cross compiler, it's perfectly possible to make haskell
binaries on platforms without GHCs. `windows ++ unix` seems good enough
for now.

Also don't default `hydraPlatforms` to `platforms`. The former must be a
list of systems (strings), but the latter is a list of systems or
patterns.

(cherry picked from commit 65e24f2)

(cherry picked from commit abf2760)

…-18.03