build: use the system printf routines in most cases

glibc <= 2.5 would crash when passed invalid long double values,
therefore internal gnulib routines were used, essentially only by od,
to output such invalid values.  Later glibc versions don't crash,
as per https://sourceware.org/bugzilla/show_bug.cgi?id=4586
and subsequently od was adjusted to use the system printf routines
through the use of the ftoastr module with commit v8.7-22-ga71c22f.
Consequently our testing of this feature was moot, and use of
the gnulib printf replacement for printf(1), od(1) and error(3) etc.
was redundant.

* configure.ac (gl_printf_safe): Unset so that we don't check that
"nan" is output for these long double values.
* tests/misc/od-float.sh: Adjust all existing checks to fail if od
exits with failure status (like crashing for example).  Add a new case
for one of the problematic invalid long double values for x86_64.
We only check that od exits successfully at present, which may change
if https://sourceware.org/bugzilla/show_bug.cgi?id=17661 is resolved.