Fix __builtin_signbit for ppcf128 type

Function__builtin_signbit returns wrong value for type ppcf128 on big endian
machines. This patch fixes how value is generated in that case.

Patch by Aleksandar Beserminji.

Differential Revision: http://reviews.llvm.org/D14149



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@252307 91177308-0d34-0410-b5e6-96231b3b80d8