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/log/gdbexec.c

@@ -28,7 +28,7 @@
 /**
  * Attaches GDB temporarily, to do something like print a variable.
  */
-privileged int(gdbexec)(const char *cmd) {
+relegated int(gdbexec)(const char *cmd) {
   struct StackFrame *bp;
   int pid, ttyin, ttyout;
   intptr_t continuetoaddr;

libc/log/log_exit.c

@@ -19,6 +19,6 @@
 #include "libc/log/log.h"
 #include "libc/runtime/runtime.h"
 
-privileged wontreturn void _log_exit(int exitcode) {
+wontreturn void _log_exit(int exitcode) {
   _Exitr(exitcode);
 }