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.

1 files changed, 8 insertions, 2 deletions

diff --git a/lib/std/os/linux/riscv64.zig b/lib/std/os/linux/riscv64.zig
index 88b5c70fbc..7ab814fa71 100644
--- a/lib/std/os/linux/riscv64.zig
+++ b/lib/std/os/linux/riscv64.zig
@@ -151,8 +151,8 @@ pub const dev_t = usize;
 pub const blkcnt_t = isize;
 
 pub const timeval = extern struct {
-    tv_sec: time_t,
-    tv_usec: i64,
+    sec: time_t,
+    usec: i64,
 };
 
 pub const Flock = extern struct {
@@ -223,3 +223,9 @@ pub const Stat = extern struct {
 pub const Elf_Symndx = u32;
 
 pub const VDSO = struct {};
+
+/// TODO
+pub const ucontext_t = void;
+
+/// TODO
+pub const getcontext = {};