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/runtime/getsymbol.c

@@ -0,0 +1,56 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2023 Justine Alexandra Roberts Tunney                              │
+│                                                                              │
+│ Permission to use, copy, modify, and/or distribute this software for         │
+│ any purpose with or without fee is hereby granted, provided that the         │
+│ above copyright notice and this permission notice appear in all copies.      │
+│                                                                              │
+│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
+│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
+│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
+│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
+│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
+│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
+│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
+│ PERFORMANCE OF THIS SOFTWARE.                                                │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/runtime/runtime.h"
+#include "libc/runtime/symbols.internal.h"
+
+extern _Hide struct SymbolTable *__symtab;
+
+/**
+ * Returns low index into symbol table for address.
+ *
+ * @param t if null will be auto-populated only if already open
+ * @return index or -1 if nothing found
+ */
+noinstrument privileged int __get_symbol(struct SymbolTable *t, intptr_t a) {
+  // we need privileged because:
+  //   kprintf is privileged and it depends on this
+  // we don't want function tracing because:
+  //   function tracing depends on this function via kprintf
+  unsigned l, m, r, n, k;
+  if (!t && __symtab) {
+    t = __symtab;
+  }
+  if (t) {
+    l = 0;
+    r = n = t->count;
+    k = a - t->addr_base;
+    while (l < r) {
+      m = (l + r) >> 1;
+      if (t->symbols[m].y < k) {
+        l = m + 1;
+      } else {
+        r = m;
+      }
+    }
+    if (l < n && t->symbols[l].x <= k && k <= t->symbols[l].y) {
+      return l;
+    }
+  }
+  return -1;
+}