std: change return type of wasiCwd

`fd_t` is not declared on freestanding so returning a `Dir` causes an error.

lib/std/fs.zig

@@ -231,17 +231,17 @@ pub fn renameW(old_dir: Dir, old_sub_path_w: []const u16, new_dir: Dir, new_sub_
 /// On POSIX targets, this function is comptime-callable.
 pub fn cwd() Dir {
     if (builtin.os.tag == .windows) {
-        return Dir{ .fd = os.windows.peb().ProcessParameters.CurrentDirectory.Handle };
+        return .{ .fd = os.windows.peb().ProcessParameters.CurrentDirectory.Handle };
     } else if (builtin.os.tag == .wasi) {
-        return std.options.wasiCwd();
+        return .{ .fd = std.options.wasiCwd() };
     } else {
-        return Dir{ .fd = os.AT.FDCWD };
+        return .{ .fd = os.AT.FDCWD };
     }
 }
 
-pub fn defaultWasiCwd() Dir {
+pub fn defaultWasiCwd() std.os.wasi.fd_t {
     // Expect the first preopen to be current working directory.
-    return .{ .fd = 3 };
+    return 3;
 }
 
 /// Opens a directory at the given path. The directory is a system resource that remains

lib/std/std.zig

@@ -203,7 +203,7 @@ pub const Options = struct {
     enable_segfault_handler: bool = debug.default_enable_segfault_handler,
 
     /// Function used to implement `std.fs.cwd` for WASI.
-    wasiCwd: fn () fs.Dir = fs.defaultWasiCwd,
+    wasiCwd: fn () os.wasi.fd_t = fs.defaultWasiCwd,
 
     /// The current log level.
     log_level: log.Level = log.default_level,

src/main.zig

@@ -45,11 +45,11 @@ pub const std_options = .{
 pub const panic = crash_report.panic;
 
 var wasi_preopens: fs.wasi.Preopens = undefined;
-pub fn wasi_cwd() fs.Dir {
+pub fn wasi_cwd() std.os.wasi.fd_t {
     // Expect the first preopen to be current working directory.
     const cwd_fd: std.os.fd_t = 3;
     assert(mem.eql(u8, wasi_preopens.names[cwd_fd], "."));
-    return .{ .fd = cwd_fd };
+    return cwd_fd;
 }
 
 pub fn getWasiPreopen(name: []const u8) Compilation.Directory {