Expose stable sort algorithm to gcc_sort_r and add vec::stablesort

Expose stable sort algorithm to gcc_sort_r and add vec::stablesort

This makes it possible to apply GCCs stable sort algorithm to vec<>
and also use it with the qsort_r compatible interface.

2021-06-10  Richard Biener  <rguenther@suse.de>

	* system.h (gcc_stablesort_r): Declare.
	* sort.cc (gcc_sort_r): Support stable sort.
	(gcc_stablesort_r): Define.
	* vec.h (vec<>::stablesort): Add.

(cherry-picked from commit 367f52dcc24045b072aeb26bc301a2980b39241f)