Fix off-by-one error in stack delta lookup#185
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When the current address is a return address, we need to subtract one to get the real call instruction. Anecdotally, fixing this bug seems to substantially improve unwinding for Rust aarch64 binaries compiled in debug mode. The reason for this is that our heuristic for getting the frame pointer doesn't work in function epilogues, and in debug builds, you often have a call instruction immediately followed by the function epilogue. So due to this off-by-one error, we'll think we're in the epilogue and not correctly unwind the fp. It seems much less common in release builds, probably because it is much more unusual to have a call immediately followed by the function epilogue (because the optimizer will replace that sequence by a tail call).
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Cherry-pick of open-telemetry#1027 -- see there for PR description