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

[Rafael Auler via llvm-commits]

rafaelauler wrote:

My work so far has been mainly to steer this in the direction of not breaking the normal build, but I can't verify that the actual goal (the standalone build) is working at all -- in fact, I'm not even sure why is that a thing, and why a builder that is only interested in packaging BOLT wouldn't just build it with:

```
  cmake llvm-project/llvm -DLLVM_ENABLE_PROJECTS= "bolt"
```
  
and then
```
  ninja install-bolt
```
  
I tried that and got a install folder with the bare minimum to run BOLT:

```
(CMAKE_INSTALL_PREFIX set to /home/rafaelauler/install/)
/home/rafaelauler/install/lib
/home/rafaelauler/install/lib/libbolt_rt_instr.a
/home/rafaelauler/install/lib/libbolt_rt_hugify.a
/home/rafaelauler/install/bin
/home/rafaelauler/install/bin/llvm-bolt
/home/rafaelauler/install/bin/perf2bolt
/home/rafaelauler/install/bin/llvm-boltdiff
/home/rafaelauler/install/bin/merge-fdata
/home/rafaelauler/install/bin/llvm-bolt-heatmap
```
  
So what exactly are we getting by having a standalone CMakeFile?

I also tried running cmake directly on llvm-project/bolt with this patch applied, but it fails with:

```
CMake Error at CMakeLists.txt:38 (find_package):
  Could not find a package configuration file provided by "LLVM" with any of
  the following names:

    LLVMConfig.cmake
    llvm-config.cmake

  Add the installation prefix of "LLVM" to CMAKE_PREFIX_PATH or set
  "LLVM_DIR" to a directory containing one of the above files.  If "LLVM"
  provides a separate development package or SDK, be sure it has been
  installed.
```



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