[PowerPC][compiler-rt][builtins]Fix __fixunstfti builtin on PowerPC

__fixunstfti converts a long double (IBM double-double) to an unsigned 128 bit
integer.  This patch enables it to handle a previously unhandled case in which
a negative low double may impact the result of the conversion.

Collaborated with @masoud.ataei and @renenkel.
Patch By: Baptiste Saleil

Differential Revision: https://reviews.llvm.org/D69193