Add cmpxchgi(), cmpxchgl() and cpu_relax() functions that we can use
as simple primitives to build spinlocks out of.
PR-URL: https://github.com/libuv/libuv/pull/197
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>
[NB: This is a back-port of commit a3c3b37 from the v1.x branch.]
