Validate privileged code relationships

- Work towards improving non-optimized build support
- Introduce MODE=zero which is -O0 without ASAN/UBSAN
- Use system GCC when ~/.cosmo.mk has USE_SYSTEM_TOOLCHAIN=1
- Have package.com check .privileged code doesn't call non-privileged

libc/calls/prctl.c

@@ -31,7 +31,7 @@
  *
  * @raise ENOSYS on non-Linux
  */
-privileged int prctl(int operation, ...) {
+int prctl(int operation, ...) {
   int rc;
   va_list va;
   intptr_t a, b, c, d;