[llvm] [BOLT][AArch64] Add call relaxation pass (PR #173952)

[Rafael Auler via llvm-commits]

rafaelauler wrote:

@labrinea thanks for the ideas -- since Maksim's implementation has been rather well battle-tested (in prod for 6mo), I'll go ahead and merge it for open-source users to try it (it should be gated on --relax-exp). 

I'll add some PRs next week to put more gates on it (such as disabling it if you exceed 128MB in hot code).

I feel the implementation that you suggested would be working more in a direction of completely replacing the old long jmp, since it is a more general implementation. Or you can later try directly modifying Maksim's implementation to fix any issues that you face in other cases -- is this a reasonable path forward? Let me know

https://github.com/llvm/llvm-project/pull/173952