ldbl-96: Set -1 to "int sign_exponent:16"

ldbl-96: Set -1 to "int sign_exponent:16"

ieee_long_double_shape_type has

typedef union
{
  long double value;
  struct
  {
    ...
    int sign_exponent:16;
    ...
  } parts;
} ieee_long_double_shape_type;

Clang issues an error:

../sysdeps/ieee754/ldbl-96/test-totalorderl-ldbl-96.c:49:2: error: implicit truncation from 'int' to bit-field changes value from 65535 to -1 [-Werror,-Wbitfield-constant-conversion]
   49 |         SET_LDOUBLE_WORDS (ldnx, 0xffff,
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   50 |                            tests[i] >> 32, tests[i] & 0xffffffffULL);
      |

Use -1, instead of 0xffff, to silence Clang.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Reviewed-by: Sam James <sam@gentoo.org>