[compiler-rt builtins] Assert that atomic.c can be compiled correctly.

[efriedma]

Summary

The spinlock requires that lock-free operations are available; otherwise, the implementation just calls itself. As discussed in D120026.

compiler-rt/lib/builtins/atomic.c

 typedef OSSpinLock Lock;
 __inline static void unlock(Lock *l) { OSSpinLockUnlock(l); }
 /// Locks a lock.  In the current implementation, this is potentially
 /// unbounded in the contended case.
 __inline static void lock(Lock *l) { OSSpinLockLock(l); }
 static Lock locks[SPINLOCK_COUNT]; // initialized to OS_SPINLOCK_INIT which is 0
 
 #else
+_Static_assert(__atomic_always_lock_free(sizeof(uintptr_t), 0),
+               "Implementation assumes lock-free pointer-size cmpxchg");
 typedef _Atomic(uintptr_t) Lock;
 /// Unlock a lock.  This is a release operation.
 __inline static void unlock(Lock *l) {
   __c11_atomic_store(l, 0, __ATOMIC_RELEASE);
 }
 /// Locks a lock.  In the current implementation, this is potentially
 /// unbounded in the contended case.
 __inline static void lock(Lock *l) {