alignasof: port to IBM XL C 16.1 * doc/gnulib.texi (alignof): Improve doc to match implementationa better. * m4/stdalign.m4 (alignas): Do not define to _Alignas if __xlC__ claims to conform to C11, because _Alignas fails with weird diagnostics "Unexpected text %1$s encountered." * tests/test-alignasof.c (TEST_ALIGNMENT, alignas): Follow doc advice.