This patch fixes __fixunstfti builtin for PowerPC.
This builtin converts a long double (IBM double-double) to an unsigned 128 bit integer.
The patch allows the builtin to handle a previously unhandled case in which a negative low double may impact the result of the conversion.
@masoud.ataei and @renenkel helped me to fix this builtin and to generate additional tests to test the builtin using negative low doubles.