darwin-ldouble.c: Add big comment explaining exactly what is expected as a 'long...

darwin-ldouble.c: Add big comment explaining exactly what is expected as a 'long double'.

	* config/rs6000/darwin-ldouble.c: Add big comment explaining
	exactly what is expected as a 'long double'.
	(_xlqadd): When a value to be returned is representable as a
	'double', just return it directly, do not construct it using a union.
	Also, correct final fixup.
	(_xlqmul): Likewise.
	(_xlqdiv): Likewise.
	* real.c (encode_ibm_extended): Make consistent with darwin-ldouble.c.

From-SVN: r75629