Prior to this PR, gnu-config did its unpacking in the `buildPhase`,
which mean that `.override({ patches = ... })` couldn't be used to
test out patches to `gnu-config`.

This PR fixes that.

For almost a decade nixpkgs has parsed `*-*-{cygwin,msvc}` as being
ABIs for a common kernel (windows), rather than as kernels.  It
isn't really reasonable to change nixpkgs' handling at this point,
so this commit replicates the special-case hack when unparsing, in
order to ensure that `(unparse . parse)==id`.

The eminent Donald E. Knuth should be recognized as having equal
standing with such entities as IBM, Apple, and the Personal
Computer.  We should acknowledge this by including him as a "vendor".

Also, `gnu-config` recognizes `mmix-knuth-*` triples (and in fact
requires `vendor="knuth"` when `cpu="mmix"`) -- so we sort of have
to.  But we should do it anyways.

tripleFromSkeleton contains the same logic found in
tripleFromSystem, but is capable of operating on unvalidated
attrsets-of-strings.

gnu-config does not recognize `none` as a vendor -- that string
describes a *kernel*

Unfortunately gnu-config triples are, in some rare cases, sensitive
to whether the vendor was specified as `unknown` or was omitted.  In
other words, there are situations where these are not handled
exactly the same way:

- `mips-linux-gnu`
- `mips-unknown-linux-gnu`

This commit introduces `lib.systems.tripleFromSystemLossy`, which
returns `"mips-unknown-linux-gnu"` for both of the platforms above.
This is almost always what you want to use.

Since 4aa1ffa we have been shipping
reverse-engineered out-of-tree forks of gcc and binutils for the
Broadcom VC4.  Upstream for those forks chose the nonstandard
`vc4-elf` triple to identify their system.  GNU config rejects this
triple (and all `vc4-*` triples).

This commit deals with the fallout from that.

Mainly this commit ensures that we are consistent in using `vc4-elf`
instead of `vc4-none`, and makes sure that parsing->unparsing
round-trips properly.

GNU config (correctly) considers Solaris to be a BSD, and therefore
appends its version to the kernel name, which should be `solaris2`
not `solaris`.

This commit updates `lib/systems` to do the same.

Each kernel supports only a subset of the universe of possible ABIs.
This commit explicitly lists (opt-in) the allowable abis for each
kernel.  Attempting to parse a kernel with an abi which is not on
the list will fail.  This "search space pruning" is essential to
getting our gnu-config agreement tests down to a manageable set of
test cases.  It also lets us reject nonsense triples instead of
trying to handle them exactly the same way gnu-config does.

In PR #182807 we decided that big-endian PowerPC64 should default to
the new elfv2 ABI.  The implementation in that PR did the defaulting
by adding two lines in the *triple parser*, which is turning out to
be problematic; it means that our parse-then-unparse roundtrip
disagrees with gnu-config.

This commit therefore reverts just those two lines.  If the
defaulting logic needs to be moved elsewhere that should be done.

https://github.com/NixOS/nixpkgs/pull/182807/files#r1234650738

This is required for agreement with gnu-config.

gnu-config doesn't include this.

This commit fixes the logic for handling missing/unknown vendor
fields in triples, and extends it to handle mmix and microblaze
CPU-types.

This commit adds lib/tests/triples.nix, which exhaustively tests
that our platform triple parse-then-unparse round-trip agrees with
gnu-config for all inputs that it accepts (it may -- and does --
reject triples which `gnu-config` accepts).

With this commit, we now parse or reject the following identically
to gnu-config:

- aarch64-solo5-none (accept)
- aarch64-solo5-none-elf (reject)

Closes #165836.

This commit adds support for triples with a missing kernel, rather
than kernel="none".  It also adds a parser case for triples which
have a vendor and abi, but no kernel.

This allows to parse "*-unknown-elf" triples.

Closes #230160.

… needed

Commit 3f32d4541dd4a08765a307363b368471c72d0348 bumped our
gnu-config to 2023-07-31, which includes
91f6a7f616b161c25ba2001861a40e662e18c4ad "config.sub: Accept
LLVM-style $cpu-$vendor-windows-{gnu,msvc}".

That commit allows us to remove some of the windows-triple hackery,
but the part that makes "cygnus" a valid ABI (note: this is
different from the part that makes "cygwin" a kernel!) cannot (yet)
be removed.