[libstdc++][testsuite] XFAIL tests relying on long double-to-string conversions on...

[libstdc++][testsuite] XFAIL tests relying on long double-to-string conversions on broken newlib

    * lib/target-supports.exp
    (check_effective_target_newlib_broken_long_double_io): New check.
    (check_effective_target_frexpl): Likewise.

    * testsuite/21_strings/basic_string/numeric_conversions/char/stod.cc:
    XFAIL run if newlib_broken_long_double_io.
    * testsuite/21_strings/basic_string/numeric_conversions/wchar_t/stod.cc:
    Likewise.
    * testsuite/21_strings/basic_string/numeric_conversions/wchar_t/stold.cc:
    Likewise.
    * testsuite/27_io/basic_ostream/inserters_arithmetic/char/hexfloat.cc:
    Likewise.

From-SVN: r243558