[compiler-rt][builtins][PowerPC] Implemented __floattitf builtin on PowerPC

This patch implements the long double __floattitf (int128_t) method for
PowerPC -- specifically to convert a 128 bit integer into a long double
(IBM double-double).

To invoke this method, one can do so by linking against compiler-rt, via the
--rtlib=compiler-rt command line option supplied to clang.

Differential Revision: https://reviews.llvm.org/D54313/

llvm-svn: 350818