Avoid maybe-uninitialized warning in __kernel_rem_pio2

With GCC 14 on 32-bit x86 the compiler emits a maybe-uninitialized
warning:

../sysdeps/ieee754/dbl-64/k_rem_pio2.c: In function '__kernel_rem_pio2':
../sysdeps/ieee754/dbl-64/k_rem_pio2.c:364:20: error: 'fq' may be used uninitialized [-Werror=maybe-uninitialized]
  364 |           y[0] = fq[0]; y[1] = fq[1]; y[2] = fw;
      |                  ~~^~~

This is similar to the warning that is suppressed in the other branch of
the switch.  Help the compiler knowing that the variable is always
initialized, which also makes the suppression obsolete.