[Darwin, testsuite] Fix fail of gnu2x-attrs-1.c.

This test fails on targets without symbol alias support, but we don't
want to skip it entirely with the usual dg-requires, thus expect the
error on the alias line.

gcc/testsuite/ChangeLog:

2019-11-21  Iain Sandoe  <iain@sandoe.co.uk>

	* gcc.dg/gnu2x-attrs-1.c: Expect an error for the alias case
	on Darwin.

From-SVN: r278547

gcc/testsuite/ChangeLog

+2019-11-21  Iain Sandoe  <iain@sandoe.co.uk>
+
+	* gcc.dg/gnu2x-attrs-1.c: Expect an error for the alias case
+	on Darwin.
+
 2019-11-21  Jakub Jelinek  <jakub@redhat.com>
 
 	PR c++/90842

gcc/testsuite/gcc.dg/gnu2x-attrs-1.c

@@ -5,7 +5,7 @@
 
 void f (void) {};
 
-[[gnu::alias("f")]] void g (void);
+[[gnu::alias("f")]] void g (void); /* { dg-error "only weak" *-*-darwin* } */
 
 void [[gnu::alias("f")]] h (void); /* { dg-warning "ignored" } */
 /* { dg-message "that appertains to a type-specifier" "appertains" { target *-*-* } .-1 } */