libc: Define __atomic_is_lock_free.

Limited to architectures where it is actually needed by gcc for any
calls to stdatomic.h atomic_is_lock_free for now.

We should also add it to other architectures too, along with lockful
atomic r/m/w operations for sizes that can't be handled natively, but
that's a lot more work. It is also necessary for -fno-inline-atomics
but we're missing a lot of other symbols for that too, to be fixed.
For now, this should enable the OpenSSL build to complete on these
architectures again after I reverted a local change.

XXX pullup-10

branches: 1.8.14;
Add missing atomic_and_{8,16}_nv_cas.c for __sync_and_and_fetch_{1,2}.
XXX why is not only atomic_and_* symmetric unlike the others?
(in common/lib/libc/atomic/)

branches: 1.7.16;
Hook __atomic_compare_exchange_N into vax libc.

branches: 1.6.4;
Add the missing __sync_* ops to libc

branches: 1.5.8; 1.5.14;
allow inclusion of atomic ops in librump

Allow atomic ops to be built as part of libpthread.

branches: 1.3.4;
Only build atomic ops for libkern/libc.

Enable the atomic ops in userspace.

Add the .C versions for the atomic ops.