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/winerr.greg.c

@@ -35,7 +35,6 @@ privileged int64_t __winerr(void) {
   errno_t e;
   if (IsWindows()) {
     e = __dos2errno(__imp_GetLastError());
-    _npassert(e > 0);
   } else {
     e = ENOSYS;
   }