Replace the legacy __sync built-in functions with __atomic ones

Replace the legacy __sync built-in functions with __atomic ones

Last-Update: 2016-10-04
Forwarded: not-needed

libunwind uses the built-in __sync_* functions which are deprecated by GCC and
should be replaced by __atomic_* ones. See the official manuals [1].

The legacy __sync functions do not require to specify the memory order but
__atomic ones do, so we choose the strongest one: __ATOMIC_SEQ_CST.

We do this because __sync_fetch_and_add() is not supported on armel.

[1]: https://gcc.gnu.org/onlinedocs/gcc/_005f_005fsync-Builtins.html

Gbp-Pq: Topic external/libunwind
Gbp-Pq: Name legacy_built-in_sync_functions.patch