testsuite: Support { target c++20 } in tests.

gcc/testsuite/ChangeLog
2020-05-12  Jason Merrill  <jason@redhat.com>

	* lib/target-supports.exp (check_effective_target_c++20_only)
	(check_effective_target_c++20): New.

gcc/testsuite/ChangeLog

+2020-05-25  Jason Merrill  <jason@redhat.com>
+
+	* lib/target-supports.exp (check_effective_target_c++20_only)
+	(check_effective_target_c++20): New.
+
 2020-05-24  Uroš Bizjak  <ubizjak@gmail.com>
 
 	PR target/95255

gcc/testsuite/lib/target-supports.exp

@@ -9134,6 +9134,14 @@ proc check_effective_target_c++2a { } {
     return [check_effective_target_c++2a_only]
 }
 
+proc check_effective_target_c++20_only { } {
+    return [check_effective_target_c++2a_only]
+}
+
+proc check_effective_target_c++20 { } {
+    return [check_effective_target_c++2a]
+}
+
 # Check for C++ Concepts support, i.e. -fconcepts flag.
 proc check_effective_target_concepts { } {
     if [check_effective_target_c++2a] {