std.c reorganization

It is now composed of these main sections:
* Declarations that are shared among all operating systems.
* Declarations that have the same name, but different type signatures
  depending on the operating system. Often multiple operating systems
  share the same type signatures however.
* Declarations that are specific to a single operating system.
  - These are imported one per line so you can see where they come from,
    protected by a comptime block to prevent accessing the wrong one.

Closes #19352 by changing the convention to making types `void` and
functions `{}`, so that it becomes possible to update `@hasDecl` sites
to use `@TypeOf(f) != void` or `T != void`. Happily, this ended up
removing some duplicate logic and update some bitrotted feature
detection checks.

A handful of types have been modified to gain namespacing and type
safety. This is a breaking change.

Oh, and the last usage of `usingnamespace` site is eliminated.

CMakeLists.txt

@@ -404,7 +404,6 @@ set(ZIG_STAGE2_SOURCES
     lib/std/buf_map.zig
     lib/std/builtin.zig
     lib/std/c.zig
-    lib/std/c/linux.zig
     lib/std/coff.zig
     lib/std/crypto.zig
     lib/std/crypto/blake3.zig