Clean up more code

- Found some bugs in LLVM compiler-rt library
- The useless LIBC_STUBS package is now deleted
- Improve the overflow checking story even further
- Get chibicc tests working in MODE=dbg mode again
- The libc/isystem/ headers now have correctly named guards

libc/intrin/__cxa_pure_virtual.c

@@ -17,11 +17,12 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/intrin/kprintf.h"
+#include "libc/runtime/runtime.h"
 
 void __cxa_pure_virtual(void) {
 #ifndef NDEBUG
   kprintf("__cxa_pure_virtual() called\n"
           "Did you call a virtual method from a destructor?\n");
 #endif
-  __builtin_trap();
+  abort();
 }