Fix the printing of float special values with write_float in C grades.

runtime/mercury_float.c:
	When printing floats, do not append ".0" to nan, inf or -inf.

tests/hard_coded/Mmakefile:
tests/hard_coded/write_float_special.{m,exp,exp2}:
	Add a regression test for the above.

NEWS:
	Announce the above.