posix_thread.c: Restore old signal alternate stack before thread exit (#3693)

Some host environment may also create an signal alternate stack and access
it after the wasm runtime exits, the runtime should backup the stack info and
restore it before thread exits.

The issue was found in golang:
```
signal 23 received on thread with on signal stack
fatal error: non-Go code disabled signaltstack
```

core/shared/platform/common/posix/posix_thread.c

@@ -504,6 +504,8 @@ static os_thread_local_attribute bool thread_signal_inited = false;
 #if WASM_DISABLE_STACK_HW_BOUND_CHECK == 0
 /* The signal alternate stack base addr */
 static os_thread_local_attribute uint8 *sigalt_stack_base_addr;
+/* The previous signal alternate stack */
+static os_thread_local_attribute stack_t prev_sigalt_stack;
 
 /*
  * ASAN is not designed to work with custom stack unwind or other low-level
@@ -683,7 +685,9 @@ os_thread_signal_init(os_signal_handler handler)
     sigalt_stack_info.ss_sp = map_addr;
     sigalt_stack_info.ss_size = map_size;
     sigalt_stack_info.ss_flags = 0;
-    if (sigaltstack(&sigalt_stack_info, NULL) != 0) {
+    memset(&prev_sigalt_stack, 0, sizeof(stack_t));
+    /* Set signal alternate stack and save the previous one */
+    if (sigaltstack(&sigalt_stack_info, &prev_sigalt_stack) != 0) {
         os_printf("Failed to init signal alternate stack\n");
         goto fail2;
     }
@@ -729,19 +733,12 @@ fail1:
 void
 os_thread_signal_destroy()
 {
-#if WASM_DISABLE_STACK_HW_BOUND_CHECK == 0
-    stack_t sigalt_stack_info;
-#endif
-
     if (!thread_signal_inited)
         return;
 
 #if WASM_DISABLE_STACK_HW_BOUND_CHECK == 0
-    /* Disable signal alternate stack */
-    memset(&sigalt_stack_info, 0, sizeof(stack_t));
-    sigalt_stack_info.ss_flags = SS_DISABLE;
-    sigalt_stack_info.ss_size = SIG_ALT_STACK_SIZE;
-    sigaltstack(&sigalt_stack_info, NULL);
+    /* Restore the previous signal alternate stack */
+    sigaltstack(&prev_sigalt_stack, NULL);
 
     os_munmap(sigalt_stack_base_addr, SIG_ALT_STACK_SIZE);