Definen LLP64 data model for code completion

This change should not be interpreted as an intent to support MSVC which
appears to live on in VSCode IntelliSense for what's dead will never die

libc/bits/atomic_load.c

@@ -27,8 +27,21 @@
  * optimizations.
  */
 intptr_t(atomic_load)(void *p, size_t n) {
-  intptr_t x;
-  x = 0;
-  memcpy(&x, p, MAX(n, sizeof(x)));
-  return x;
+  intptr_t x = 0;
+  switch (n) {
+    case 1:
+      __builtin_memcpy(&x, p, 1);
+      return x;
+    case 2:
+      __builtin_memcpy(&x, p, 2);
+      return x;
+    case 4:
+      __builtin_memcpy(&x, p, 4);
+      return x;
+    case 8:
+      __builtin_memcpy(&x, p, 8);
+      return x;
+    default:
+      return 0;
+  }
 }

libc/bits/atomic_store.c

@@ -27,6 +27,20 @@
  * optimizations.
  */
 intptr_t(atomic_store)(void *p, intptr_t x, size_t n) {
-  memcpy(p, &x, MAX(n, sizeof(x)));
-  return x;
+  switch (n) {
+    case 1:
+      __builtin_memcpy(p, &x, 1);
+      return x;
+    case 2:
+      __builtin_memcpy(p, &x, 2);
+      return x;
+    case 4:
+      __builtin_memcpy(p, &x, 4);
+      return x;
+    case 8:
+      __builtin_memcpy(p, &x, 8);
+      return x;
+    default:
+      return 0;
+  }
 }