Add OpenBSD support for ICU library loading

[am11]

OpenBSD uses ABI versioning in SONAME rather than ICU version numbering. Use unversioned dlopen calls and probe symbol versioning instead of attempting to load versioned library names.

Contributes to #124911.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-globalization
See info in area-owners.md if you want to be subscribed.

[am11]

Hello world was working with the override:

am11@foo:~$ DOTNET_ICU_VERSION_OVERRIDE=22 th/artifacts/bin/testhost/net11.0-openbsd-Debug-x64/dotnet exec ~/projects/hw1/hw1/bin/Debug/net11.0/hw1.dll

Hello, World!
Environment.Version: 11.0.0
RuntimeInformation.OSDescription: OpenBSD 7.8 GENERIC.MP#54

This makes it work without.

[tarekgh]

Two important caveats this PR introduces

1. The manually installed ICU must be built --disable-renaming. The OpenBSD path calls FindSymbolVersion(-1, -1, -1, ...), which only matches the unversioned u_strlen. If you drop in a stock upstream ICU build (renaming enabled → u_strlen_78), dlopen succeeds but symbol resolution returns false and ICU fails to load. So a "more recent ICU" has to be packaged the OpenBSD way (unversioned symbols), not a vanilla ./configure && make build.

2. DOTNET_ICU_VERSION_OVERRIDE no longer works on OpenBSD after this PR. This is the subtle one. FindLibUsingOverride (which reads DOTNET_ICU_VERSION_OVERRIDE) lives only in the #else generic-Unix branch (lines 332-432). The new OpenBSD #elif branch never calls it. Pre-PR, OpenBSD fell through to #else, which is exactly why the author's DOTNET_ICU_VERSION_OVERRIDE=22 workaround worked. Post-PR, that env var is silently ignored on OpenBSD. So the one knob that let you force a particular major version on the Linux path is gone for OpenBSD — you'd have to use LD_LIBRARY_PATH or app-local ICU instead.