[mono] Add headers for unstable APIs

[lambdageek]

We have functions that are de-facto part of the Mono embedding API that are
used by Xamarin.iOS or Xamarin.Android that are not in a state where we want to
commit to supporting them forever in their current form.

Nonetheless, we would like to make sure that the symbols for these functions
are visible even if we otherwise compile the runtime with
-fvisisibility=hidden, and we would like to discourage declaring the
functions in the embedder's own headers.

The functions that go into the
mono/{utils,metadata,jit}/mono-private-unstable.h headers will all be marked
with MONO_API MONO_RT_EXTERNAL_ONLY but we will not guarantee that the
functions will be there from one release to the next or that they will not
change their signatures or their behaviors.

[lambdageek]

Attn: @vargaz @migueldeicaza

[lambdageek]

An example where we might want to use this header is for mono_install_load_aot_data_hook and mono_gc_init_finalizer_thread from #33736

From that PR:

• mono_install_load_aot_data_hook was already a MONO_API, but it was not in a public header. See https://github.com/xamarin/xamarin-macios/blob/master/runtime/exports.t4
• mono_gc_init_finalizer_thread was used by Xamarin.iOS for a long time without being in any public header. It's one of the reasons we have to compile Mono for XI with --disable-visibility-hidden. Make the function a proper MONO_API unconditionally - but make it do nothing if the runtime is compiled without --with-lazy-thread-creation (the default).

[lambdageek]

FYI @jkotas

[jaykrell]

Perhaps this set should be extern "C++" (with a C++ wrapper even), so that signature changes are fatal. I understand the implications -- each platform/architecture gets different names.
Alternatively, a policy of changing the name when changing signature.

[lambdageek]

Perhaps this set should be extern "C++" (with a C++ wrapper even), so that signature changes are fatal.

Problem is that some embedders like mono/tests/libtest.c or Xamarin.iOS will use dlsym to get at these symbols so mangling the name to include the types will cause us pain.

Alternatively, a policy of changing the name when changing signature.

yea that's an interesting idea. I would say changing the semantics should also require a name change (though that may be harder to catch). Maybe just bumping some numeric suffic on each function name. Nothing says unstable like mono_zombo_init_37

[jaykrell]

I understand the dlsym part, but that is not insurmountable.
The names are discoverable.
People do use GetProcAddress and dlsym with mangled names.
Some kind of "manual name mangling" is easier but not enforced.