libstdc++: Fix long double to_chars testcase [PR98384]

libstdc++: Fix long double to_chars testcase [PR98384]

The testcase was failing to compile on some targets due to its use of
the non-standard functions nextupl and nextdownl.  This patch makes the
testcase instead use the C99 function nexttowardl in an equivalent way.

libstdc++-v3/ChangeLog:

	PR libstdc++/98384
	* testsuite/20_util/to_chars/long_double.cc: Use nexttowardl
	instead of the non-standard nextupl and nextdownl.