dlsym: Do not determine caller link map if not needed

dlsym: Do not determine caller link map if not needed

Obtaining the link map is potentially very slow because it requires
iterating over all loaded objects in the current implementation.  If
the caller supplied an explicit handle (i.e., not one of the RTLD_*
constants), the dlsym implementation does not need the identity of the
caller (except in the special case of auditing), so this change
avoids computing it in that case.

Even in the minimal case (dlsym called from a main program linked with
-dl), this shows a small speedup, perhaps around five percent.  The
performance improvement can be arbitrarily large in principle (if
_dl_find_dso_for_object has to iterate over many link maps).

Change-Id: Ide5d9e2cc7ac25a0ffae8fb4c26def0c898efa29