[cmake] Add LLVM_TARGET_TRIPLE_ENV to LLVMConfig.cmake.in (#158303)

LLVM has a mechanism to switch the default target:
LLVM_TARGET_TRIPLE_ENV. For example, regression tests with lit use this
CMake variable. It matters only when building LLVM, not the other
projects. Therefore, if they are built standalone, the value of the
CMake variable should be propagated. In fact, however, it is not.
This patch fixes it.

Author: parabola94

llvm/cmake/modules/LLVMConfig.cmake.in

@@ -36,6 +36,8 @@ set(LLVM_TARGETS_WITH_JIT @LLVM_TARGETS_WITH_JIT@)
 
 set(LLVM_TARGET_TRIPLE "@LLVM_TARGET_TRIPLE@")
 
+set(LLVM_TARGET_TRIPLE_ENV "@LLVM_TARGET_TRIPLE_ENV@")
+
 set(LLVM_HOST_TRIPLE "@LLVM_HOST_TRIPLE@")
 
 set(LLVM_ABI_BREAKING_CHECKS @LLVM_ABI_BREAKING_CHECKS@)