Use more atomic handling of thread flags

This can prevent a data race on Linux with `Self_Cleanup`.

core/thread/thread.odin

@@ -272,7 +272,7 @@ create_and_start :: proc(fn: proc(), init_context: Maybe(runtime.Context) = nil,
 	t := create(thread_proc, priority)
 	t.data = rawptr(fn)
 	if self_cleanup {
-		t.flags += {.Self_Cleanup}
+		intrinsics.atomic_or(&t.flags, {.Self_Cleanup})
 	}
 	t.init_context = init_context
 	start(t)
@@ -307,7 +307,7 @@ create_and_start_with_data :: proc(data: rawptr, fn: proc(data: rawptr), init_co
 	t.user_index = 1
 	t.user_args[0] = data
 	if self_cleanup {
-		t.flags += {.Self_Cleanup}
+		intrinsics.atomic_or(&t.flags, {.Self_Cleanup})
 	}
 	t.init_context = init_context
 	start(t)
@@ -347,7 +347,7 @@ create_and_start_with_poly_data :: proc(data: $T, fn: proc(data: T), init_contex
 	mem.copy(&t.user_args[0], &data, size_of(T))
 
 	if self_cleanup {
-		t.flags += {.Self_Cleanup}
+		intrinsics.atomic_or(&t.flags, {.Self_Cleanup})
 	}
 
 	t.init_context = init_context
@@ -394,7 +394,7 @@ create_and_start_with_poly_data2 :: proc(arg1: $T1, arg2: $T2, fn: proc(T1, T2),
 	_  = copy(user_args[n:], mem.ptr_to_bytes(&arg2))
 
 	if self_cleanup {
-		t.flags += {.Self_Cleanup}
+		intrinsics.atomic_or(&t.flags, {.Self_Cleanup})
 	}
 
 	t.init_context = init_context
@@ -443,7 +443,7 @@ create_and_start_with_poly_data3 :: proc(arg1: $T1, arg2: $T2, arg3: $T3, fn: pr
 	_  = copy(user_args[n:], mem.ptr_to_bytes(&arg3))
 
 	if self_cleanup {
-		t.flags += {.Self_Cleanup}
+		intrinsics.atomic_or(&t.flags, {.Self_Cleanup})
 	}
 
 	t.init_context = init_context
@@ -494,7 +494,7 @@ create_and_start_with_poly_data4 :: proc(arg1: $T1, arg2: $T2, arg3: $T3, arg4:
 	_  = copy(user_args[n:], mem.ptr_to_bytes(&arg4))
 
 	if self_cleanup {
-		t.flags += {.Self_Cleanup}
+		intrinsics.atomic_or(&t.flags, {.Self_Cleanup})
 	}
 
 	t.init_context = init_context

core/thread/thread_windows.odin

@@ -27,7 +27,7 @@ _create :: proc(procedure: Thread_Proc, priority: Thread_Priority) -> ^Thread {
 	__windows_thread_entry_proc :: proc "system" (t_: rawptr) -> win32.DWORD {
 		t := (^Thread)(t_)
 
-		if .Joined in t.flags {
+		if .Joined in sync.atomic_load(&t.flags) {
 			return 0
 		}
 
@@ -48,9 +48,9 @@ _create :: proc(procedure: Thread_Proc, priority: Thread_Priority) -> ^Thread {
 			t.procedure(t)
 		}
 
-		intrinsics.atomic_store(&t.flags, t.flags + {.Done})
+		intrinsics.atomic_or(&t.flags, {.Done})
 
-		if .Self_Cleanup in t.flags {
+		if .Self_Cleanup in sync.atomic_load(&t.flags) {
 			win32.CloseHandle(t.win32_thread)
 			t.win32_thread = win32.INVALID_HANDLE
 			// NOTE(ftphikari): It doesn't matter which context 'free' received, right?