BIGGEST_ALIGNMENT vs g++ compat test expectation on aix ?

[Olivier Hainque <hainque@adacore.com>]

Working on a collect2 related patch resubmission for ppc-aix, I
stumbled on regressions in the c++ series for a problem unrelated to
my change. Not sure how you'd typically deal with these, so providing
the datapoints here.

tmpdir-g++.dg-struct-layout-1/t027 and a couple of other tests failed
from constructs like

    struct S2661 {
	union{v16sf b[31];Tal2short c __attribute__((aligned (4)));}a;
	float d;
    };

    struct S2661 s2661;
    struct S2661 a2661[5];

    info.als = __alignof__ (s2661);
    if (&a2661[3] & (info.als - 1)) FAIL ...

the alignment check is up to that of v16sf (64bytes), and this
occasionaly fails because csects are "only" aligned in accordance with

   rs6000.h:#define BIGGEST_ALIGNMENT 128

The discrepancy is visible straight from the assembly output
for e.g. t027_y (out of the unchanged compiler as well):

        .csect .data[RW],4
                        ^^^
        .align 6
              ^^^
a2661:
        .space 10240

Olivier