darwin, testsuite fix PR27221

The test can't succeed on 32b powerpc Darwin since the
ABI demands a different result to the tested one.

2019-05-22  Iain Sandoe  <iain@sandoe.co.uk>

	PR testsuite/27221
	* g++.dg/ext/alignof2.C: XFAIL for 32bit Darwin.

From-SVN: r271505