fix msghdr and cmsghdr on non-Linux targets, document musl behaviour

The following assertions fail on non-Linux platforms after c0c2010535
which inserted padding based on musl definitions. This padding only
exists on musl to workaround a discrepancy betweeen the POSIX API and
Linux ABI, and is incorrect on other POSIX operating systems.

This change makes the padding musl-only, and documents the reason it
exists. With this change, the assertions pass on Linux and FreeBSD
targets. The corresponding definitions on other targets line up with the
POSIX and FreeBSD ones, so they should work there too.

```zig
const std = @import("std");
const assert = std.debug.assert;
const msghdr = std.c.msghdr;
const cmsghdr = std.c.cmsghdr;

const c = @cImport({
    @cInclude("sys/socket.h");
});

comptime {
    assert(@offsetOf(msghdr, "iovlen") == @offsetOf(c.msghdr, "msg_iovlen"));
    assert(@offsetOf(msghdr, "controllen") == @offsetOf(c.msghdr, "msg_controllen"));
    assert(@offsetOf(msghdr, "control") == @offsetOf(c.msghdr, "msg_control"));
    assert(@offsetOf(msghdr, "flags") == @offsetOf(c.msghdr, "msg_flags"));

    assert(@sizeOf(msghdr) == @sizeOf(c.msghdr));

    assert(@offsetOf(cmsghdr, "len") == @offsetOf(c.cmsghdr, "cmsg_len"));
    assert(@offsetOf(cmsghdr, "level") == @offsetOf(c.cmsghdr, "cmsg_level"));

    assert(@sizeOf(cmsghdr) == @sizeOf(c.cmsghdr));
}
```

lib/std/c.zig

@@ -4186,18 +4186,45 @@ pub const msghdr = switch (native_os) {
     else => void,
 };
 
+/// There are several instances of struct fields that POSIX defines as either int or socklen_t, but
+/// on Linux are size_t. glibc ignores POSIX, and uses the Linux kernel's definitions. musl on the
+/// other hand aims to be POSIX-ly correct, and defines those fields in a manner aligning with
+/// POSIX.
+///
+/// musl works around this incompatibility between the 64-bit Linux ABI and the POSIX specification
+/// by adding padding fields on either side depending on host endianness:
+///
+///     #if __LONG_MAX > 0x7fffffff && __BYTE_ORDER == __BIG_ENDIAN
+///         int __pad2;
+///     #endif
+///         socklen_t msg_controllen;
+///     #if __LONG_MAX > 0x7fffffff && __BYTE_ORDER == __LITTLE_ENDIAN
+///         int __pad2;
+///     #endif
+///
+/// To emulate this quirk of musl, the MuslOnlyPadding field is used in these structs
+///
+///     pad0: MuslOnlyPadding(.big) = 0,
+///     msg_controllen: socklen_t,
+///     pad1: MuslOnlyPadding(.little) = 0,
+///
+/// On 32-bit and non-musl systems, these fields will be zero sized, and ignored.
+fn MuslOnlyPadding(endian: std.builtin.Endian) type {
+    return if (builtin.abi.isMusl() and @sizeOf(usize) == 8 and native_endian == endian) u32 else u0;
+}
+
 /// https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/sys_socket.h.html
 const posix_msghdr = extern struct {
     name: ?*sockaddr,
     namelen: socklen_t,
     iov: [*]iovec,
-    pad0: if (@sizeOf(usize) == 8 and native_endian == .big) u32 else u0 = 0,
+    pad0: MuslOnlyPadding(.big) = 0,
     iovlen: u32,
-    pad1: if (@sizeOf(usize) == 8 and native_endian == .little) u32 else u0 = 0,
+    pad1: MuslOnlyPadding(.little) = 0,
     control: ?*anyopaque,
-    pad2: if (@sizeOf(usize) == 8 and native_endian == .big) u32 else u0 = 0,
+    pad2: MuslOnlyPadding(.big) = 0,
     controllen: socklen_t,
-    pad3: if (@sizeOf(usize) == 8 and native_endian == .little) u32 else u0 = 0,
+    pad3: MuslOnlyPadding(.little) = 0,
     flags: u32,
 };
 
@@ -4226,13 +4253,13 @@ const posix_msghdr_const = extern struct {
     name: ?*const sockaddr,
     namelen: socklen_t,
     iov: [*]const iovec_const,
-    pad0: if (@sizeOf(usize) == 8 and native_endian == .big) u32 else u0 = 0,
+    pad0: MuslOnlyPadding(.big) = 0,
     iovlen: u32,
-    pad1: if (@sizeOf(usize) == 8 and native_endian == .little) u32 else u0 = 0,
+    pad1: MuslOnlyPadding(.little) = 0,
     control: ?*const anyopaque,
-    pad2: if (@sizeOf(usize) == 8 and native_endian == .big) u32 else u0 = 0,
+    pad2: MuslOnlyPadding(.big) = 0,
     controllen: socklen_t,
-    pad3: if (@sizeOf(usize) == 8 and native_endian == .little) u32 else u0 = 0,
+    pad3: MuslOnlyPadding(.little) = 0,
     flags: u32,
 };
 
@@ -4276,9 +4303,9 @@ pub const cmsghdr = switch (native_os) {
 };
 
 const posix_cmsghdr = extern struct {
-    pad0: if (@sizeOf(usize) == 8 and native_endian == .big) u32 else u0 = 0,
+    pad0: MuslOnlyPadding(.big) = 0,
     len: socklen_t,
-    pad1: if (@sizeOf(usize) == 8 and native_endian == .little) u32 else u0 = 0,
+    pad1: MuslOnlyPadding(.little) = 0,
     level: c_int,
     type: c_int,
 };