gas: deprecate .vtable_entry and .vtable_inherit

These ELF directives were a GNU extension to support the C++ ABI
that was in use before the introduction of the generic C++ ABI in
GCC-3.0.  They're obsolete now and many newer targets simply
do not provide the back-end support for them.

Flag them as deprecated in the NEWS file, and remove the documentation
for them.  Warn (once) if the directive is used in a source file.