On illumos (and Solaris) there is, by design, only one libc.

Solaris/illumos is multi-lib, so you can't rely on an arbitrary
executable to give you the correct dynamic linker. Besides, it's
always the same path.

- Adds `illumos` to the `Target.Os.Tag` enum. A new function,
  `isSolarish` has been added that returns true if the tag is either
  Solaris or Illumos. This matches the naming convention found in Rust's
  `libc` crate[1].
- Add the tag wherever `.solaris` is being checked against.
- Check for the C pre-processor macro `__illumos__` in CMake to set the
  proper target tuple. Illumos distros patch their compilers to have
  this in the "built-in" set (verified with `echo | cc -dM -E -`).

  Alternatively you could check the output of `uname -o`.

Right now, both Solaris and Illumos import from `c/solaris.zig`. In the
future it may be worth putting the shared ABI bits in a base file, and
mixing that in with specific `c/solaris.zig`/`c/illumos.zig` files.

[1]: https://github.com/rust-lang/libc/tree/6e02a329a2a27f6887ea86952f389ca11e06448c/src/unix/solarish

The 5.11 in uname is not something that is ever updated. There is no
versioning of the illumos system in general. Illumos prefers to rely
on feature detection.

I can't say what Solaris does these days as I do not work at Oracle;
so I left it alone.