Disable various parts of -thread handling for 5.x

[dra27]

Implements a fix for an issue reported in Discuss.

The original example, given a.ml:

let x = Thread.self ()

is that this command (despite being incorrect) works in 3.x/4.x:

$ ocamlfind ocamlopt -I +unix -I +threads -package threads -linkpkg unix.cmxa threads.cmxa a.ml
ocamlfind: [WARNING] Package `threads': Linking problems may arise because of the missing -thread or -vmthread switch

but in 5.x:

$ ocamlfind ocamlopt -I +unix -I +threads -package threads -linkpkg unix.cmxa threads.cmxa a.ml
File "a.ml", line 1:
Error: Cannot find file /home/dra/.opam/fix-threads-5.1.1/lib/ocaml/threads.cmxa

This command isn't directly fixable, as the fix to ensure that threads.cmxa is linked from /home/dra/.opam/fix-threads-5.1.1/lib/ocaml/threads/threads.cmxa will then result in double-linking.

However, if we consider the correct invocation, which for ocamlfind should always involve -thread to set ocamlfind's machinery into the correct mode:

$ ocamlfind ocamlopt -package threads -linkpkg -thread a.ml -verbose
Effective set of compiler predicates: pkg_unix,pkg_threads,autolink,mt,mt_posix,native
+ ocamlopt.opt -verbose -thread -I /home/dra/.opam/fix-threads-5.1.1/lib/ocaml/unix /home/dra/.opam/fix-threads-5.1.1/lib/ocaml/unix/unix.cmxa /home/dra/.opam/fix-threads-5.1.1/lib/ocaml/threads/threads.cmxa a.ml

In 5.x, this is linking without warning, but the invocation is incorrect as it's relying on the deprecated in ocamlopt -thread option and effectively ignoring the META file for threads shipped with OCaml 5.

With this PR, that then becomes:

Effective set of compiler predicates: pkg_unix,pkg_threads,autolink,mt,mt_posix,native
+ ocamlopt.opt -verbose -I /home/dra/.opam/fix-threads-5.1.1/lib/ocaml/unix -I /home/dra/.opam/fix-threads-5.1.1/lib/ocaml/threads /home/dra/.opam/fix-threads-5.1.1/lib/ocaml/unix/unix.cmxa /home/dra/.opam/fix-threads-5.1.1/lib/ocaml/threads/threads.cmxa a.ml

and we can see the correct set of predicates, combined with standard -I and .cmxa parameters for packages. The command works with or without the -thread argument to ocamlfind, the flag now controls whether the mt and mt_posix predicates are true. My recollection when preparing ocaml/ocaml#11007 was that the mt and mt_posix predicates were not used outside of findlib itself (i.e. that there are not (m)any third-party META files which uses these two predciates)