[CMake] Use llvm-tblgen from NATIVE LLVM build when cross-compiling

Summary:
When cross-compiling LLDB, we want to use llvm-tblgen built for the
host, not the target.

Reviewers: compnerd, sgraenitz

Subscribers: mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D57194

llvm-svn: 352235