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/islinux.c

@@ -21,7 +21,7 @@
 #include "libc/runtime/runtime.h"
 #include "libc/sysv/consts/pr.h"
 
-privileged bool __is_linux_2_6_23(void) {
+bool __is_linux_2_6_23(void) {
 #ifdef __x86_64__
   int rc;
   if (!IsLinux()) return false;