[libstdc++] [testsuite] xfail double-prec from_chars for ldbl

When long double is wider than double, but from_chars is implemented
in terms of double, tests that involve the full precision of long
double are expected to fail.  Mark them as such on aarch64-*-vxworks.


for  libstdc++-v3/ChangeLog

	* testsuite/20_util/from_chars/4.cc: Skip long double test06
	on aarch64-vxworks.
	* testsuite/20_util/to_chars/long_double.cc: Xfail run on
	aarch64-vxworks.

(cherry picked from commit e383fc69)

libstdc++-v3/testsuite/20_util/from_chars/4.cc

@@ -18,6 +18,7 @@
 // <charconv> is supported in C++14 as a GNU extension
 // { dg-do run { target c++14 } }
 // { dg-add-options ieee }
+// { dg-additional-options "-DSKIP_LONG_DOUBLE" { target aarch64-*-vxworks* } }
 
 #include <charconv>
 #include <string>
@@ -354,7 +355,7 @@ test06()
 {
   test_max_mantissa<float, unsigned long>();
   test_max_mantissa<double, unsigned long long>();
-#ifdef __GLIBCXX_TYPE_INT_N_0
+#if defined __GLIBCXX_TYPE_INT_N_0 && !defined SKIP_LONG_DOUBLE
   test_max_mantissa<long double, unsigned __GLIBCXX_TYPE_INT_N_0>();
 #endif
 }

libstdc++-v3/testsuite/20_util/to_chars/long_double.cc

@@ -34,6 +34,10 @@
 // more portable and robust to differences in system printf behavior.
 // { dg-xfail-run-if "Non-conforming printf (see PR98384)" { *-*-solaris* *-*-darwin* } }
 
+// On systems that use double-precision from_chars for long double,
+// this is expected to fail.
+// { dg-xfail-run-if "from_chars limited to double-precision" { aarch64-*-vxworks* } }
+
 // { dg-require-effective-target ieee_floats }
 // { dg-require-effective-target size32plus }
 // { dg-require-cmath "" }