[PowerPC][compiler-rt][builtins]Add __fixtfti builtin on PowerPC

Implements __fixtfti builtin for PowerPC. This builtin converts a
long double (IBM double-double) to a signed int128. The conversion relies on
the unsigned conversion of the absolute value of the long double.

Tests included for both positive and negative long doubles.

Patch By: Baptiste Saleil

Differential Revision: https://reviews.llvm.org/D69730