target_is_non_stop_p and sync targets

gdb.base/maint-target-async-off.exp fails if you test against
gdbserver with "maint set target-non-stop on" forced.

  (gdb) run
  Starting program: build/gdb/testsuite/outputs/gdb.base/maint-target-async-off/maint-target-async-off

  Breakpoint 1, main () at src/gdb/testsuite/gdb.base/maint-target-async-off.c:21
  21        return 0;
  (gdb) FAIL: gdb.base/maint-target-async-off.exp: continue until exit (timeout)

Above, GDB just stopped listening to stdin.

Basically, GDB assumes that a target working in non-stop mode
operation also supports async mode; it's a requirement.  GDB
misbehaves badly otherwise, and even hits failed assertions.

Fix this by making target_is_non_stop_p return false if async is off.

gdb/ChangeLog:

	* target.c (target_always_non_stop_p): Also check whether the
	target can async.

Change-Id: I7e52e1061396a5b9b02ada462f68a14b76d68974

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2021-03-26  Pedro Alves  <[email protected]>
+
+	* target.c (target_always_non_stop_p): Also check whether the
+	target can async.
+
 2021-03-26  Tom Tromey  <[email protected]>
 
 	* dwarf2/read.c (dwarf2_read_debug_names)

gdb/target.c

@@ -4370,10 +4370,11 @@ target_always_non_stop_p (void)
 bool
 target_is_non_stop_p ()
 {
-  return (non_stop
-	  || target_non_stop_enabled == AUTO_BOOLEAN_TRUE
-	  || (target_non_stop_enabled == AUTO_BOOLEAN_AUTO
-	      && target_always_non_stop_p ()));
+  return ((non_stop
+	   || target_non_stop_enabled == AUTO_BOOLEAN_TRUE
+	   || (target_non_stop_enabled == AUTO_BOOLEAN_AUTO
+	       && target_always_non_stop_p ()))
+	  && target_can_async_p ());
 }
 
 /* See target.h.  */