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/intrin/strsignal_r.c

@@ -36,7 +36,7 @@
  * @asyncsignalsafe
  * @threadsafe
  */
-char *strsignal_r(int sig, char buf[hasatleast 15]) {
+privileged char *strsignal_r(int sig, char buf[hasatleast 15]) {
   int i;
   char *p;
   const char *s;