Add STB_GNU_UNIQUE symbol binding definition

Red Hat created STB_GNU_UNIQUE to handle certain special cases relating
to dynamically loading C++ DSOs[1].

We don't (currently) have support for STB_GNU_UNIQUE, but ought to
reserve the value in ELFNN_ST_BIND. This will also be used by an
upcoming ELF Tool Chain import.

[1] https://www.redhat.com/archives/posix-c++-wg/2009-August/msg00002.html

MFC after: 1 week
Sponsored by: The FreeBSD Foundation