[Pal/Linux] Block async signals on thread exiting

After the Linux PAL frees the thread's signal stack and TCB but before
the thread is actually destroyed by Linux, an arriving async signal on
this thread will result in a segfault. This commit simply blocks all
async signals before proceeding with freeing thread's resources.

Pal/src/host/Linux/db_threading.c

@@ -173,6 +173,7 @@ noreturn void _DkThreadExit (void)
     PAL_TCB_LINUX* tcb = get_tcb_linux();
     PAL_HANDLE handle = tcb->handle;
 
+    block_async_signals(true);
     if (tcb->alt_stack) {
         stack_t ss;
         ss.ss_sp    = NULL;