[cmake] Only build enabled targets

Summary:
When attempting to build llvm-bolt with `-DLLVM_ENABLE_TARGETS="X86"`, I
encountered an error:

```
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)
```

The issue is that the `llvm-bolt/src/Target/AArch64` subdirectory is
added by CMake unconditionally. The LLVM project, on the other hand,
only adds the subdirectories that are enabled, by using a `foreach` loop
over `LLVM_TARGETS_TO_BUILD`. Copying that same loop, from
`llvm/lib/Target/CMakeLists.txt`, to this project avoids the error.

(cherry picked from FBD15030236)