elf: Add _dl_find_object function

It can be used to speed up the libgcc unwinder, and the internal
_dl_find_dso_for_object function (which is used for caller
identification in dlopen and related functions, and in dladdr).

_dl_find_object is in the internal namespace due to bug 28503.
If libgcc switches to _dl_find_object, this namespace issue will
be fixed.  It is located in libc for two reasons: it is necessary
to forward the call to the static libc after static dlopen, and
there is a link ordering issue with -static-libgcc and libgcc_eh.a
because libc.so is not a linker script that includes ld.so in the
glibc build tree (so that GCC's internal -lc after libgcc_eh.a does
not pick up ld.so).

It is necessary to do the i386 customization in the
sysdeps/x86/bits/dl_find_object.h header shared with x86-64 because
otherwise, multilib installations are broken.

The implementation uses software transactional memory, as suggested
by Torvald Riegel.  Two copies of the supporting data structures are
used, also achieving full async-signal-safety.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

include/link.h

@@ -211,6 +211,9 @@ struct link_map
 				       freed, ie. not allocated with
 				       the dummy malloc in ld.so.  */
     unsigned int l_ld_readonly:1; /* Nonzero if dynamic section is readonly.  */
+    unsigned int l_find_object_processed:1; /* Zero if _dl_find_object_update
+					       needs to process this
+					       lt_library map.  */
 
     /* NODELETE status of the map.  Only valid for maps of type
        lt_loaded.  Lazy binding sets l_nodelete_active directly,