Fix CMakeLists.txt to allow building when LLVM backend is configured …

[hrtan99]

…other than x86 and arm.

Although llvm-mctoll currently only support x86 and arm, it should be built correctly when the backend of llvm project is configured of architectures other than x86 and arm.

Due to the following codes in mctoll, the linker will report undefined symbol for the MC layer components of other machine arch.

  // Initialize targets and assembly printers/parsers.
  llvm::InitializeAllTargets();
  llvm::InitializeAllTargetInfos();
  llvm::InitializeAllTargetMCs();
  llvm::InitializeAllDisassemblers();

We can use ${LLVM_TARGETS_TO_BUILD} in CMakeLists file to avoid the error, though it includes unnecessary components of other architectures.

[aaronsm]

LGTM

[bharadwajy]

There is no value in building llvm-mctoll for targets other than those it intends to support. Configuring as such in the publicly available repo would indicate that it is supported for targets other than x64 and ARM. Additionally, as is pointed out this change would include unnecessary components in the resulting binary and the needless complexity of adding ${LLVM_TARGETS_TO_BUILD} for no additional benefit.

The change in this PR can be made locally by anyone that wishes to build it on unsupported targets - and does so with an understanding.