[lldb][Process/FreeBSDKernelCore] Rework plugin destruction (#188426)

Destroy the plugin classes similar to `ProcessElfCore`, another process
plugin derived from `PostMortemProcess` class. After clearing thread
list, invoke `Finalize()` to cleanup resources properly. `Finalize()`
will call `DoDestroy()` which releases `m_kvm` via `kvm_close()`.

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Signed-off-by: Minsoo Choo <minsoochoo0122@proton.me>
GitOrigin-RevId: 0cefe6358f38d99e9e70cb242af5b5f8e9fe4e32