Fix missing version info in host binaries on Unix

[elinor-fung]

Fix #34505

With this:

> strings artifacts/bin/linux-x64.Release/corehost/*.so | grep '@(#'
@(#)Version 42.42.42.42424 @Commit: 805cd002a7e5dd4647efae50a6b5d16f713da23e
@(#)Version 42.42.42.42424 @Commit: 805cd002a7e5dd4647efae50a6b5d16f713da23e
@(#)Version 42.42.42.42424 @Commit: 805cd002a7e5dd4647efae50a6b5d16f713da23e

The three occurrences are libhostfxr, libhostpolicy, and libnethost. dotnet also has the version as expected and apphost does not have the version as expected.

[lpereira]

LGTM, although it might be a good idea to use CMake's SOVERSION property in shared libraries, too.

[elinor-fung]

Hm, yeah, looks like we don't use SOVERSION / VERSION right now. Just read a bit about it - probably worth looking into. Not sure how that plays with the current versioning through resources we do on Windows though.

[janvorli]

@lpereira so you are suggesting that we should consider adding version to the filename of the libraries?

[lpereira]

On Unix, it's customary to have the ABI version in the library name (e.g. libc.so.6 for ABI version 6).

It's not easy to change now because that's a published interface, but it would have been easier to, for instance, look for libhostfxr.so.2 and assume it has the new-style interface rather than opening libhostfxr.so and probing for symbols.

Symbolic links can be used to point to the latest version, too. So we probably don't even need hostfxr to locate hostpolicy, we can just link -- and not even look ourselves for it with dlopen() and friends -- with libhostpolicy.so.5, and let the dynamic loader do the work of finding the latest version of that. (For instance, that could be /usr/lib64/dotnet/libhostpolicy.so.5 -> /usr/lib64/dotnet/libhostpolicy.so.5.0.0, if we used the SOVERSION and VERSION to match the .NET version -- and if .NET 5.0.1 is released, we just patch the symlink and next time something else starts, it's already using the new version.)

These changes would make .NET look less alien in Unix, IMHO.