Disable the 'nextafter' portions of these tests on PPC when using 128-bit...

Disable the 'nextafter' portions of these tests on PPC when using 128-bit doubles because the 'nextafter' call doesn't work right. Reviewed as https://reviews.llvm.org/D62384. Thanks to Xing Xue for the patch, and Hubert for the explanation.

llvm-svn: 363740