[CMake] Only set LLVM_DEFAULT_TARGET_TRIPLE to LLVM_HOST_TRIPLE when native target is enabled

This is for case when native target like X86 is not in LLVM_TARGETS_TO_BUILD.
Right now LLVM_DEFAULT_TARGET_TRIPLE is set to LLVM_HOST_TRIPLE even when native target is not enabled,
As a result, many lit tests will fail because default_triple is set for lit test but not enabled when build LLVM.

Reviewed By: smeenai

Differential Revision: https://reviews.llvm.org/D134972