Bad choice of aliased symbol name in symbolizer

Originally reported on Google Code with ID 249

```
We symbolize "textdomain" from libc as "__textdomain", and strdup as "__GI___strdup".

For textdomain, we need to pick global symbol ahead of a local one, if they point to
the same address.

For strdup, __GI___strdup is local, so the above would take care of it. But there is
also
global "__strdup"
global weak "strdup"

Not sure what to do. We could prioritize symbols that don't start with underscore.
Would it make sense to prioritize weak symbols? What do other symbolizer libraries
do in this case?

```

Reported by `eugenis@google.com` on 2013-11-29 11:49:35