Revert "thread: simplify and remove useless return in spawn (#8621)"

This reverts commit 77cb45f59f.

Zig's error return traces will point to the return token if they
happen to occur, so having multiple return statements makes those
stack traces really helpful. This destroys debuggability.

lib/std/Thread.zig

@@ -360,13 +360,15 @@ pub fn spawn(comptime startFn: anytype, context: SpawnContextType(@TypeOf(startF
             MainFuncs.posixThreadMain,
             thread_obj.data.memory.ptr,
         );
-        return switch (err) {
-            0 => thread_obj,
-            os.EAGAIN => error.SystemResources,
+        switch (err) {
+            0 => return thread_obj,
+            os.EAGAIN => return error.SystemResources,
             os.EPERM => unreachable,
             os.EINVAL => unreachable,
-            else => os.unexpectedErrno(err),
-        };
+            else => return os.unexpectedErrno(err),
+        }
+
+        return thread_obj;
     }
 
     var guard_end_offset: usize = undefined;