Misc. std.Io fixes for WASI and Emscripten

- Corrects WASI `UTIME_*` definitions now that the libc build has been
  fixed (see previous commit), and adds the corresponding definitions
  for Emscripten which were missing.
- Fixes `dirReadUnimplemented()`, which didn't compile.
- Prevents a dependency on `pthread_kill` from being pulled in in
  single-threaded Emscripten builds, where it isn't defined.

With these changes, Emscripten can now participate in juicy main.

lib/std/Io/Threaded.zig

@@ -2024,6 +2024,7 @@ fn groupConcurrent(
 
 fn groupAwait(userdata: ?*anyopaque, type_erased: *Io.Group, initial_token: *anyopaque) Io.Cancelable!void {
     _ = initial_token; // we need to load `token` *after* the group finishes
+    if (builtin.single_threaded) unreachable; // nothing to await
     const t: *Threaded = @ptrCast(@alignCast(userdata));
     const g: Group = .{ .ptr = type_erased };
 
@@ -2082,6 +2083,7 @@ fn groupAwait(userdata: ?*anyopaque, type_erased: *Io.Group, initial_token: *any
 
 fn groupCancel(userdata: ?*anyopaque, type_erased: *Io.Group, initial_token: *anyopaque) void {
     _ = initial_token;
+    if (builtin.single_threaded) unreachable; // nothing to cancel
     const t: *Threaded = @ptrCast(@alignCast(userdata));
     const g: Group = .{ .ptr = type_erased };
 
@@ -2152,6 +2154,7 @@ fn await(
     result_alignment: Alignment,
 ) void {
     _ = result_alignment;
+    if (builtin.single_threaded) unreachable; // nothing to await
     const t: *Threaded = @ptrCast(@alignCast(userdata));
     const future: *Future = @ptrCast(@alignCast(any_future));
 
@@ -2218,6 +2221,7 @@ fn cancel(
     result_alignment: Alignment,
 ) void {
     _ = result_alignment;
+    if (builtin.single_threaded) unreachable; // nothing to cancel
     const t: *Threaded = @ptrCast(@alignCast(userdata));
     const future: *Future = @ptrCast(@alignCast(any_future));
 
@@ -4959,7 +4963,7 @@ fn dirReadUnimplemented(userdata: ?*anyopaque, dir_reader: *Dir.Reader, buffer:
     _ = userdata;
     _ = dir_reader;
     _ = buffer;
-    return error.Unimplemented;
+    return error.Unexpected;
 }
 
 const dirRealPathFile = switch (native_os) {
@@ -13224,7 +13228,7 @@ fn processSpawnPath(userdata: ?*anyopaque, dir: Dir, options: process.SpawnOptio
 }
 
 const processSpawn = switch (native_os) {
-    .wasi, .ios, .tvos, .visionos, .watchos => processSpawnUnsupported,
+    .wasi, .emscripten, .ios, .tvos, .visionos, .watchos => processSpawnUnsupported,
     .windows => processSpawnWindows,
     else => processSpawnPosix,
 };

lib/std/c.zig

@@ -97,6 +97,7 @@ pub const off_t = switch (native_os) {
 pub const timespec = switch (native_os) {
     .linux => linux.timespec,
     .emscripten => emscripten.timespec,
+    // lib/libc/include/wasm-wasi-musl/__struct_timespec.h
     .wasi => extern struct {
         sec: time_t,
         nsec: isize,
@@ -115,16 +116,18 @@ pub const timespec = switch (native_os) {
                 @as(wasi.timestamp_t, @intCast(ts.nsec));
         }
 
+        // lib/libc/include/wasm-wasi-musl/__header_sys_stat.h
+
         /// For use with `utimensat` and `futimens`.
         pub const NOW: timespec = .{
             .sec = 0,
-            .nsec = 0x3fffffff,
+            .nsec = -1,
         };
 
         /// For use with `utimensat` and `futimens`.
         pub const OMIT: timespec = .{
             .sec = 0,
-            .nsec = 0x3ffffffe,
+            .nsec = -2,
         };
     },
     // https://github.com/SerenityOS/serenity/blob/0a78056453578c18e0a04a0b45ebfb1c96d59005/Kernel/API/POSIX/time.h#L17-L20
@@ -7007,6 +7010,7 @@ pub const time_t = switch (native_os) {
     .linux => linux.time_t,
     .emscripten => emscripten.time_t,
     .haiku, .dragonfly => isize,
+    // lib/libc/include/wasm-wasi-musl/__typedef_time_t.h
     // https://github.com/SerenityOS/serenity/blob/b98f537f117b341788023ab82e0c11ca9ae29a57/Kernel/API/POSIX/sys/types.h#L47
     else => i64,
 };

lib/std/os/emscripten.zig

@@ -729,6 +729,7 @@ pub const sockaddr = c.sockaddr;
 
 pub const blksize_t = i32;
 pub const nlink_t = u32;
+// https://github.com/emscripten-core/emscripten/blob/946ab574ae39401b51e75cd5257d894ae732ab54/system/lib/libc/musl/arch/emscripten/bits/alltypes.h#L140
 pub const time_t = i64;
 pub const mode_t = u32;
 pub const off_t = i64;
@@ -765,9 +766,24 @@ pub const stack_t = extern struct {
     size: usize,
 };
 
+// https://github.com/emscripten-core/emscripten/blob/946ab574ae39401b51e75cd5257d894ae732ab54/system/lib/libc/musl/arch/emscripten/bits/alltypes.h#L284
 pub const timespec = extern struct {
     sec: time_t,
     nsec: isize,
+
+    // https://github.com/emscripten-core/emscripten/blob/d72d7226f4733af8ff993dec70198cf09a24142d/system/lib/libc/musl/include/sys/stat.h#L77-L78
+
+    /// For use with `utimensat` and `futimens`.
+    pub const NOW: timespec = .{
+        .sec = 0,
+        .nsec = 0x3fffffff,
+    };
+
+    /// For use with `utimensat` and `futimens`.
+    pub const OMIT: timespec = .{
+        .sec = 0,
+        .nsec = 0x3ffffffe,
+    };
 };
 
 pub const timezone = extern struct {