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

test/libc/runtime/prog/ftraceasm.c

@@ -18,6 +18,7 @@
 ╚─────────────────────────────────────────────────────────────────────────────*/
 
 void ftrace_hook(void);
-void _start(void) {
+
+privileged void _start(void) {
   ftrace_hook();
 }