Implement __builtin_signbitl for PowerPC

PowerPC uses the special PPC_FP128 type for long double on Linux, which is
composed of two 64-bit doubles. The higher-order double (which contains the
overall sign) comes first, and so the __builtin_signbitl implementation
requires special handling to extract the sign bit.

Fixes PR20691.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@216341 91177308-0d34-0410-b5e6-96231b3b80d8