libctf: fix assertion failure with no system qsort_r

libctf: fix assertion failure with no system qsort_r

If no suitable qsort_r is found in libc, we fall back to an
implementation in ctf-qsort.c.  But this implementation routinely calls
the comparison function with two identical arguments. The comparison
function that ensures that the order of output types is stable is not
ready for this, misinterprets it as a type appearing more that once (a
can-never-happen condition) and fails with an assertion failure.

Fixed, audited for further instances of the same failure (none found)
and added a no-qsort test to my regular testsuite run.

libctf/:
	PR libctf/30013
	* ctf-dedup.c (sort_output_mapping): Inputs are always equal to
	themselves.