GNU-specific and missing on many platforms, see
https://www.gnu.org/software///gnulib/manual/html_node/strtold_005fl.html

If it does go missing, the only problem seems to be some minor changes
in long double printing code, failing two tests:

```
 ______________________________ test_str_roundtrip ______________________________

      @pytest.mark.skipif(IS_MUSL,
                          reason="test flaky on musllinux")
      @pytest.mark.skipif(LD_INFO.precision + 2 >= repr_precision,
                          reason="repr precision not enough to show eps")
      def test_str_roundtrip():
          # We will only see eps in repr if within printing precision.
          o = 1 + LD_INFO.eps
  >       assert_equal(np.longdouble(str(o)), o, "str was %s" % str(o))
  E       AssertionError:
  E       Items are not equal: str was 1.0000000000000000001
  E        ACTUAL: np.longdouble('1.0')
  E        DESIRED: np.longdouble('1.0000000000000000001')

  o          = np.longdouble('1.0000000000000000001')

  numpy/_core/tests/test_longdouble.py:43: AssertionError
  ____________________ TestRealScalars.test_dragon4_interface ____________________

  self = <numpy._core.tests.test_scalarprint.TestRealScalars object at 0x7f5804791df0>

      def test_dragon4_interface(self):
          tps = [np.float16, np.float32, np.float64]
          # test is flaky for musllinux on np.float128
          if hasattr(np, 'float128') and not IS_MUSL:
              tps.append(np.float128)

          fpos = np.format_float_positional
          fsci = np.format_float_scientific

          for tp in tps:
              # test padding
              assert_equal(fpos(tp('1.0'), pad_left=4, pad_right=4), "   1.    ")
              assert_equal(fpos(tp('-1.0'), pad_left=4, pad_right=4), "  -1.    ")
  >           assert_equal(fpos(tp('-10.2'),
                           pad_left=4, pad_right=4), " -10.2   ")
  E           AssertionError:
  E           Items are not equal:
  E            ACTUAL: ' -10.1999999999999992895'
  E            DESIRED: ' -10.2   '

  fpos       = <function format_float_positional at 0x7f580b047700>
  fsci       = <function format_float_scientific at 0x7f580b047670>
  self       = <numpy._core.tests.test_scalarprint.TestRealScalars object at 0x7f5804791df0>
  tp         = <class 'numpy.longdouble'>
  tps        = [<class 'numpy.float16'>, <class 'numpy.float32'>, <class 'numpy.float64'>, <class 'numpy.longdouble'>]

  numpy/_core/tests/test_scalarprint.py:276: AssertionError
  =========================== short test summary info ============================
  FAILED numpy/_core/tests/test_longdouble.py::test_str_roundtrip - AssertionError:
  Items are not equal: str was 1.0000000000000000001
   ACTUAL: np.longdouble('1.0')
   DESIRED: np.longdouble('1.0000000000000000001')
  FAILED numpy/_core/tests/test_scalarprint.py::TestRealScalars::test_dragon4_interface - AssertionError:
  Items are not equal:
   ACTUAL: ' -10.1999999999999992895'
   DESIRED: ' -10.2   '
```

put GNU-source above locale.h

fix issue in numpyos.c