Same as #2: llvm-build: error: invalid target to enable: 'AArch64' (not in project)

## Steps to reproduce

```
git clone https://github.com/avast/llvm
cd llvm
mkdir build && cd build
cmake ..
```

## Output

```
-- Constructing LLVMBuild project information
CMake Error at CMakeLists.txt:679 (message):
Unexpected failure executing llvm-build: Usage: llvm-build [options]

llvm-build: error: invalid target to enable: 'AArch64' (not in project)

-- Configuring incomplete, errors occurred!
See also "/home/kyc/cs/projects/retdec-llvm/build/CMakeFiles/CMakeOutput.log".
See also "/home/kyc/cs/projects/retdec-llvm/build/CMakeFiles/CMakeError.log".
```

## Notes

* It seems that the fix 329d350 was somehow reverted while merging at 82632c5.
* After applying the changes in 329d350, the output becomes:
```
-- Constructing LLVMBuild project information
-- Linker detection: GNU ld
-- Targeting X86
CMake Error at cmake/modules/LLVM-Config.cmake:175 (message):
Target X86 is not in the set of libraries.
Call Stack (most recent call first):
cmake/modules/AddLLVM.cmake:548 (llvm_map_components_to_libnames)
cmake/modules/AddLLVM.cmake:626 (llvm_add_library)
tools/lto/CMakeLists.txt:19 (add_llvm_library)

-- Configuring incomplete, errors occurred!
See also "/home/kyc/cs/projects/retdec-llvm/build/CMakeFiles/CMakeOutput.log".
See also "/home/kyc/cs/projects/retdec-llvm/build/CMakeFiles/CMakeError.log".
```