[libunwind][AIX] Handle VAPI-based return addresses in stack unwinding for LLU (#209280)

In AIX's implementation of LLU (Live Library Update), the caller of a
Virtual API (VAPI) interface instead calls VAPI glue and the
implementation for a VAPI function is entered from the VAPI glue. If a
VAPI is not already active on the thread, the VAPI calls the
implementation of the VAPI function with the link register (LR) value
set to a return address in the VAPI glue. In this case, the LR (return
address) value on entry to the VAPI glue is saved in the VAPI control
block. This PR checks whether the return address in a stack frame falls
within the VAPI address range. If it does, the unwinder retrieves the LR
value from the VAPI control block and uses it as the return address
during stack unwinding. In addition, before transferring control to a
landing pad, this PR executes the VAPI return glue to clear the VAPI
control block if the VAPI glue was used.

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Co-authored-by: Hubert Tong <hubert.reinterpretcast@gmail.com>
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GitOrigin-RevId: 8aa808457c6aeba25ec0e17139c3fcab59923c3c