ppc32: math failures - logb_downward (1) gives wrong result

[Andreas Jaeger]

Build current trunk (as of yesterday) on s390 with gcc 4.7, I see a 
couple of failures in the math testsuite:

$ cat math/test-float.out
testing float (without inline functions)
Failure: Test: logb_downward (1.000e+0) == +0
Result:
  is:         -0.00000000000000000000e+00  -0x0.00000000000000000000p+0
  should be:   0.00000000000000000000e+00   0x0.00000000000000000000p+0
  difference:  0.00000000000000000000e+00   0x0.00000000000000000000p+0
  ulp       :  0.0000
  max.ulp   :  0.0000

Test suite completed:
   3885 test cases plus 3441 tests for exception flags executed.
   1 errors occurred.

$ cat math/test-double.out
testing double (without inline functions)
Failure: Test: logb_downward (1.000e+0) == +0
Result:
  is:         -0.00000000000000000000e+00  -0x0.00000000000000000000p+0
  should be:   0.00000000000000000000e+00   0x0.00000000000000000000p+0
  difference:  0.00000000000000000000e+00   0x0.00000000000000000000p+0
  ulp       :  0.0000
  max.ulp   :  0.0000

Test suite completed:
   4152 test cases plus 3690 tests for exception flags executed.
   1 errors occurred.

$ cat math/test-ldouble.out
testing long double (without inline functions)
Failure: Test: logb_downward (1.000e+0) == +0
Result:
  is:         -0.00000000000000000000e+00  -0x0.00000000000000000000p+0
  should be:   0.00000000000000000000e+00   0x0.00000000000000000000p+0
  difference:  0.00000000000000000000e+00   0x0.00000000000000000000p+0
  ulp       :  0.0000
  max.ulp   :  0.0000

Test suite completed:
   5326 test cases plus 4468 tests for exception flags executed.
   1 errors occurred.

Can anybody reproduce this and investigate, please?

Andreas
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