Add support for symbol table in .com files

This change fixes minor bugs and adds a feature, which lets us store the
ELF symbol table, inside the ZIP directory. We use the path /zip/.symtab
which can be safely removed using a zip editing tool, to make the binary
smaller after compilation. This supplements the existing method of using
a separate .com.dbg file, which is still supported. The intent is people
don't always know that it's a good idea to download the debug file. It's
not great having someone's first experience be a crash report, that only
has numbers rather than symbols. This will help fix that!

libc/log/backtrace2.c

@@ -65,7 +65,6 @@ static int PrintBacktraceUsingAddr2line(int fd, const struct StackFrame *bp) {
   char buf[kBacktraceBufSize], *argv[kBacktraceMaxFrames];
 
   if (!(debugbin = FindDebugBinary())) {
-    ShowHint("can't find .com.dbg file try setting COMDBG");
     return -1;
   }