Getting callee in payload exit function in the case of indirect call

[tylergu]

In the case of indirect call with register-related addressing mode, SPI finds the callee by fetching saved register value from stack and computes the effective callee address. But this is not possible in the payload exit function as the saved registers are consumed by the original function call.

[tylergu]

We now save callee information in the pre-instrumentation function, and retrieve it in the post-instrumentation function.