[cmake] Don't build Native llvm-config when cross compiling if passed by user.

Summary:
Rename LLVM_CONFIG_EXE to LLVM_CONFIG_PATH, and avoid building it if
passed in by user.  This is the same way CLANG_TABLEGEN and
LLVM_TABLEGEN are handled, e.g., when -DLLVM_OPTIMIZED_TABLEGEN=ON is
passed.

Differential Revision: https://reviews.llvm.org/D41806

llvm-svn: 323053