Fix maybe-uninitialized error on powerpc

The build has been failing on powerpc64le-linux-gnu with GCC 10
due to a maybe-uninitialized error:

../sysdeps/ieee754/dbl-64/mpa.c:875:6: error: ‘w.e’ may be used
uninitialized in this function [-Werror=maybe-uninitialized]
  875 |   EY -= EX;
      |      ^~

The warning is thrown because when __inv is called by __dvd *y is not
initialized and if t == 0 before calling __dbl_mp, EY will stay
uninitialized, as the function does not touch it in this case.

However, since t will be set to 1/t before calling __dbl_mp, t == 0 will
never happen, so we can instruct the compiler to ignore this case, which
suppresses the warning.

Tested on powerpc64le.

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>