gdb/doublest.c: Use frexpl rather than ldfrexp.

gdb/doublest.c: Use frexpl rather than ldfrexp.

Most modern systems have frexpl and gnulib provides an implementation
for those that don't, so use it instead of the generic but inaccurate
ldfrexp.

gdb/ChangeLog:

2013-06-21  Will Newton  <will.newton@linaro.org>

	* doublest.c (ldfrexp): Remove function.
	(convert_doublest_to_floatformat): Call frexpl instead of
	ldfrexp.