Abstract out REAL_VALUE_TYPE streaming.

Abstract out REAL_VALUE_TYPE streaming.

In upcoming patches I will contribute code to stream out frange's as
well as vrange's.  This patch abstracts out the REAL_VALUE_TYPE
streaming into their own functions, so that they may be used elsewhere.

gcc/ChangeLog:

	* data-streamer.cc (bp_pack_real_value): New.
	(bp_unpack_real_value): New.
	* data-streamer.h (bp_pack_real_value):  New.
	(bp_unpack_real_value): New.
	* tree-streamer-in.cc (unpack_ts_real_cst_value_fields): Use
	bp_unpack_real_value.
	* tree-streamer-out.cc (pack_ts_real_cst_value_fields): Use
	bp_pack_real_value.