[llvm] [BOLT] Accept PLT fall-throughs as valid traces (PR #129481)

[Paschalis Mpeis via llvm-commits]

https://github.com/paschalis-mpeis commented:

Hey folks,

While`callcont-fallthru.s` is an x86-specific test, it can be attempted on non-x86 targets that do build the x86 target. And currently our AArch64 BOLT buildbot fails (it's internal for now, but we are working on making it public eventually).

@aaupov, can we `REQUIRE` this test to run only on `x86_64-linux`?

---

It's actually an assertion on `link_fdata.py` that causes the test to fail:
```bash
AssertionError: ERROR: symbol puts at plt is not defined in binary
```

Interestingly, on some other AArch64 host I've checked the assertion is not raised and somehow the tests passes. Still, it is not meaningful for non-x86 targets.

https://github.com/llvm/llvm-project/pull/129481