c++, testsuite - require alias support for a test.

The test for pr84497 uses a facility that requires alias support from
the target, and therefore fails on targets without.  Add a suitable 
dg-require statement.

2019-05-31  Iain Sandoe  <iain@sandoe.co.uk>

	* g++.dg/cpp0x/pr84497.C: Require alias support.

From-SVN: r271801

gcc/testsuite/ChangeLog

 2019-05-31  Iain Sandoe  <iain@sandoe.co.uk>
 
+	* g++.dg/cpp0x/pr84497.C: Require alias support.
+
+2019-05-31  Iain Sandoe  <iain@sandoe.co.uk>
+
 	* gcc.target/i386/falign-functions-3.c: Adjust align syntax
 	and label for Darwin.
 	* gcc.target/i386/attr-aligned-2.c: Adjust align syntax for

gcc/testsuite/g++.dg/cpp0x/pr84497.C

 // PR 84497 mismatch with thread constructor fn weakness
 // { dg-do compile { target c++11 } }
 // { dg-require-weak "" }
+// { dg-require-alias "" }
 // { dg-skip-if "No .weak" { { hppa*-*-hpux* } && { ! lp64 } } }
 
 struct Base