[llvm] [BOLT] Enable standalone build (PR #97130)

[Tristan Ross via llvm-commits]

RossComputerGuy wrote:

> ```
> [5/6](1) Linking CXX static library lib/libbolt_rt_instr.a
> [6/6](0) Linking CXX static library /Volumes/Work/s/w/ir/x/w/llvm_build/lib/libbolt_rt_instr_osx.a
> ```

This doesn't feel right, I would expect it to say this:
```
[5/6](1) Linking CXX static library lib/libbolt_rt_instr.a
[6/6](0) Linking CXX static library lib/libbolt_rt_instr_osx.a
```

But it's not. It's linking to a full path. But the CMake changes responsible for this look the same as `libbolt_rt_instr.a`.


https://github.com/llvm/llvm-project/pull/97130