[builtins][libc++] Allow building atomics after latest merge

These projects now check for __atomic_always_lock_free on uintptr_t which
returns false for purecap until the pull request
#721 is merged.

compiler-rt/lib/builtins/atomic.c

@@ -92,8 +92,11 @@ __inline static void lock(Lock *l) { OSSpinLockLock(l); }
 static Lock locks[SPINLOCK_COUNT]; // initialized to OS_SPINLOCK_INIT which is 0
 
 #else
+#ifndef __CHERI_PURE_CAPABILITY__
+// FIXME: needs https://github.com/CTSRD-CHERI/llvm-project/pull/721
 _Static_assert(__atomic_always_lock_free(sizeof(uintptr_t), 0),
                "Implementation assumes lock-free pointer-size cmpxchg");
+#endif
 typedef _Atomic(uintptr_t) Lock;
 /// Unlock a lock.  This is a release operation.
 __inline static void unlock(Lock *l) {

libcxx/include/atomic

@@ -1408,6 +1408,13 @@ _Tp __cxx_atomic_fetch_xor(__cxx_atomic_lock_impl<_Tp>* __a,
 
 template<typename _Tp> struct __cxx_is_always_lock_free {
     enum { __value = __atomic_always_lock_free(sizeof(_Tp), 0) }; };
+#ifdef __CHERI_PURE_CAPABILITY__
+// FIXME: can be removed after https://github.com/CTSRD-CHERI/llvm-project/pull/721 lands
+template<typename _Tp> struct __cxx_is_always_lock_free<intptr_t> { enum { __value = 2 == ATOMIC_POINTER_LOCK_FREE }; };
+template<typename _Tp> struct __cxx_is_always_lock_free<uintptr_t> { enum { __value = 2 == ATOMIC_POINTER_LOCK_FREE }; };
+template<typename _Tp> struct __cxx_is_always_lock_free<_Tp*> { enum { __value = 2 == ATOMIC_POINTER_LOCK_FREE }; };
+template<> struct __cxx_is_always_lock_free<std::nullptr_t> { enum { __value = 2 == ATOMIC_POINTER_LOCK_FREE }; };
+#endif
 
 #else