[SystemZ] Do not support __float128

As of rev. 268898, clang supports __float128 on SystemZ.  This seems to
have been in error.  GCC has never supported __float128 on SystemZ,
since the "long double" type on the platform is already IEEE-128. (GCC
only supports __float128 on platforms where "long double" is some other
data type.)

For compatibility reasons this patch removes __float128 on SystemZ
again.  The test case is updated accordingly.

llvm-svn: 348247