elf: Support dlvsym within libc.so

This commit adds a new _dl_open_hook entry for dlvsym and implements the
function using the existing dl_lookup_symbol_x function supplied by the
dynamic loader.

A new hook variable, _dl_open_hook2, is introduced, which should make
this change suitable for backporting: For old statically linked
binaries, __libc_dlvsym will always return NULL.