CMake configuration requires LLVM AArch64 target

[JohanEngelen]

The CMake configuration of LLVM with BOLT fails in case only the X86 target is built:

# ... while following the standard BOLT build instructions ...
cmake ../llvm -DLLVM_TARGETS_TO_BUILD=X86

fails with

CMake Error at cmake/modules/AddLLVM.cmake:559 (add_dependencies):
  The dependency target "AArch64CommonTableGen" of target
  "LLVMBOLTTargetAArch64" does not exist.
Call Stack (most recent call first):
  cmake/modules/AddLLVM.cmake:607 (llvm_add_library)
  tools/llvm-bolt/src/Target/AArch64/CMakeLists.txt:1 (add_llvm_library)

A simple if-statement in CMake script is probably enough to fix this. I'm not familiar enough with LLVM's CMake scripts to quickly fix this though.

Thanks.

[JohanEngelen]

(note: the easy workaround for now is to build with AArch64 target enabled too.)