Replace sync builtins with atomic builtins

The old __sync builtins have been deprecated for a long time now in
favor of the __atomic builtins following the C++11/C11 memory model.
This patch converts libgfortran to use the modern __atomic builtins.

At the same time I weakened the consistency to relaxed for
incrementing and decrementing the counter, and acquire-release when
decrementing to check whether the counter is 0 and the unit can be
freed.  This is similar to e.g. std::shared_ptr in C++.

Regtested on x86_64-pc-linux-gnu.

libgfortran/ChangeLog:

2018-11-22  Janne Blomqvist  <jb@gcc.gnu.org>

	* acinclude.m4 (LIBGFOR_CHECK_ATOMIC_FETCH_ADD): Rename and test
	presence of atomic builtins instead of sync builtins.
	* configure.ac (LIBGFOR_CHECK_ATOMIC_FETCH_ADD): Call new test.
	* io/io.h (inc_waiting_locked): Use __atomic_fetch_add.
	(predec_waiting_locked): Use __atomic_add_fetch.
	(dec_waiting_unlocked): Use __atomic_fetch_add.
	* config.h.in: Regenerated.
	* configure: Regenerated.
        * Makefile.in: Regenerated.

From-SVN: r266367