[XRay][x86_64] Define a tail exit trampoline.

Summary:
We define a new trampoline that's a hybrid between the exit and entry
trampolines with the following properties:

- Saves all of the callee-saved registers according to the x86_64
  calling conventions.
- Indicate to the log handler function being called that this is a
  function exit event.

This fixes a bug that is a result of not saving enough of the register
states, and that the log handler is clobbering registers that would be
used by the function being tail-exited into manifesting as runtime
errors.

Reviewers: rSerge, echristo, majnemer

Subscribers: mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D26020

llvm-svn: 285787

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