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

[Tristan Ross via llvm-commits]

RossComputerGuy wrote:

@aaupov Test plan script (set `$src` to whatever path you want):
```sh
cp -r cmake $src/cmake
cp -r bolt $src/bolt
cp -r third-party $src/third-party
cp -r llvm $src/llvm
cd $src/build
cmake .. $src/bolt
make # or ninja
```

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