[NativeAOT] correctly initalize CONTEXT before failing fast (#81010)

[NativeAOT] correctly initalize CONTEXT before failing fast (#81010)

* [NativeAOT] correctly initalize CONTEXT before failing fast

* Switch from using GetThreadContext to RtlCaptureContext.

* Add a better explination of why the function is unimplmented on Unix.

* Update src/coreclr/nativeaot/Runtime/PalRedhawk.h

Co-authored-by: Vladimir Sadov <vsadov@microsoft.com>
* Respond to feedback: unconditionally set CONTEXT_CONTROL

* Respond to feedback: consolidate setting of ContextFlags.

* On second thought, don't add a second layer of ifdef nesting.

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Co-authored-by: Vladimir Sadov <vsadov@microsoft.com>