[cmake] Export LLVM_HOST_TRIPLE in the LLVMConfig.cmake

This is referenced in several of the cmake files that are part of an llvm install and it is also useful by downstream components such as onnx-mlir.

Differential Revision: https://reviews.llvm.org/D106686

llvm/cmake/modules/LLVMConfig.cmake.in

@@ -36,6 +36,8 @@ set(LLVM_TARGETS_WITH_JIT @LLVM_TARGETS_WITH_JIT@)
 
 set(TARGET_TRIPLE "@TARGET_TRIPLE@")
 
+set(LLVM_HOST_TRIPLE "@LLVM_HOST_TRIPLE@")
+
 set(LLVM_ABI_BREAKING_CHECKS @LLVM_ABI_BREAKING_CHECKS@)
 
 set(LLVM_ENABLE_WARNINGS @LLVM_ENABLE_WARNINGS@)