Do not use const attribute for nan functions (bug 23277) [committed]

[Florian Weimer]

* Joseph Myers:

> +static int
> +do_test (void)
> +{
> +  char buf[2] = { '2', 0 };
> +  float a = nanf (buf);
> +  buf[0] = '3';
> +  float b = nanf (buf);
> +  return memcmp (&a, &b, sizeof (float)) == 0;
> +}

By the way, there's TEST_COMPARE_BLOB now, which can simplify
debugging of test failures because the failure message includes the
blobs that differ.