[libcxxabi/runtimes] Set LLVM_HOST_TRIPLE in runtimes build

This allows tests to tell if they're running natively.

Those tests are libcxxabi/test/native/arm-linux-eabi.
Which were running on Linaro's bots but became unsupported
when we switched to the runtimes build.

Reviewed By: #libc_abi, phosek

Differential Revision: https://reviews.llvm.org/D113663

runtimes/CMakeLists.txt

@@ -111,10 +111,11 @@ include(HandleLLVMOptions)
 
 find_package(Python3 REQUIRED COMPONENTS Interpreter)
 
-if (NOT LLVM_DEFAULT_TARGET_TRIPLE)
-  include(GetHostTriple)
-  get_host_triple(LLVM_DEFAULT_TARGET_TRIPLE)
-endif()
+# Host triple is used by tests to check if they are running natively.
+include(GetHostTriple)
+get_host_triple(LLVM_HOST_TRIPLE)
+set(LLVM_DEFAULT_TARGET_TRIPLE "${LLVM_HOST_TRIPLE}" CACHE STRING
+  "Default target for which LLVM will generate code.")
 
 option(LLVM_INCLUDE_TESTS "Generate build targets for the runtimes unit tests." ON)
 option(LLVM_INCLUDE_DOCS "Generate build targets for the runtimes documentation." ON)