Port bootstrap build on OpenBSD

[am11]

Contributes to #124911.

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[am11]

These path updates in src/native/libs are compatible with dotnet/arcade#16990 and the real OpenBSD system where pkg_add icu4c openssl etc. install libs and headers.

[am11]

With dotnet/arcade#16990, #129124, #129103 and #129078, we can build runtime native src/coreclr/build-runtime.sh and with -component paltests on the OpenBSD system and get all PAL tests passing.

Also cross-build succeeds with these changes and hello world works with coreclr as well as NativeAOT. OpenBSD has similarities with Android, OSX and FreeBSD, so not much "new" here just needed to run a series of trail and error with AI to get the right balance without significant amount of new code.

[janvorli]

Why do we need to enable this? Is W^X broken on OpenBSD?

[am11]

Short: One or two PAL tests were failing on OpenBSD due to W^X restrictions, so I remounted the /home filesystem with wxallowed and added this linker flag to make them pass.

Long:
This is OpenBSD-specific. Unlike Linux or FreeBSD, OpenBSD enforces W^X strictly and requires both a binary opt-in (-z wxneeded) and a filesystem opt-in (wxallowed) for W+X mappings to be permitted.

The reason for -z wxneeded is that parts of the PAL/JIT may need to create mappings that are temporarily W+X.

With wxneeded, the kernel allows those mappings only when the process is running on a filesystem mounted with wxallowed. If it is not, the mapping request fails at runtime when that code path is reached (i.e., when JIT/codegen tries to allocate executable memory). It does not affect process startup by itself.

So in practice we expect two modes:

• On systems with wxallowed: app will use W+X mappings when needed.
• On systems without wxallowed: user will set DOTNET_EnableWriteXorExecute=0, and the app will avoid W+X mappings entirely.

In a follow-up, for systems without wxallowed, we could detect this automatically on OpenBSD using filesystem mount flags, for example:

bool EnableWriteXorExecute(void)
{
    struct statfs buf;
    return statfs(minipal_getexepath(), &buf) == 0 &&
       (buf.f_flags & MNT_WXALLOWED);
}

(I had too many patches in flight and this was getting long, so I skipped doing this right away)

[janvorli]

Looking at the usages of the _REF one, it seems it is not worth having two separate macros. How about naming it MCONTEXT_FROM_NATIVE or something like that so that it is clearer what the macro does, and passing address of ctx in the few cases where you have added the _REF one usage?