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

[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