Improve crash backtrace reliability

We're now able to pretty print a C++ backtrace upon crashing in pretty
much any runtime execution scenario. The default pledge sandbox policy
on Linux is now to return EPERM. If you call pledge and have debugging
functions linked (e.g. GetSymbolTable) then the symbol table shall get
loaded before any security policy is put in place. This change updates
build/bootstrap/fixupobj too and fixes some other sneaky build errors.

libc/testlib/testmain.c

@@ -94,6 +94,12 @@ dontasan int main(int argc, char *argv[]) {
   struct Dll *e;
   struct TestAspect *a;
 
+  if (errno) {
+    tinyprint(2, "error: the errno variable was contaminated by constructors\n",
+              NULL);
+    return 1;
+  }
+
   __ubsan_strict = true;
   __log_level = kLogInfo;
   GetOpts(argc, argv);