[libcxxabi/runtimes] Set LLVM_HOST_TRIPLE in runtimes build

[DavidSpickett]

Summary

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.

[phosek]

runtimes/CMakeLists.txt

113–118

We can simplify this by getting the host triple unconditionally.

[phosek]

runtimes/CMakeLists.txt

113–118

In llvm/CMakeLists.txt, we cache LLVM_DEFAULT_TARGET_TRIPLE and use LLVM_HOST_TRIPLE as the default value, see https://github.com/llvm/llvm-project/blob/2ca45adf24d8ede45b9f600d13a4b4f7005e13af/llvm/CMakeLists.txt#L707. We could use the same approach here.

[DavidSpickett]

• Get host triple unconditionally
• Set default triple to host triple by default

[phosek]

LGTM

runtimes/CMakeLists.txt

 # Avoid checking whether the compiler is working.
 set(LLVM_COMPILER_CHECKED ON)
 
 # Handle common options used by all runtimes.
 include(AddLLVM)
 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.
+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)
 option(LLVM_ENABLE_SPHINX "Use Sphinx to generate the runtimes documentation." OFF)
 
 # Use libtool instead of ar if you are both on an Apple host, and targeting Apple.
 if(CMAKE_HOST_APPLE AND APPLE)
   include(UseLibtool)