Fix GCC libc++abi build

llvm-svn: 286783
diff --git a/libcxxabi/CMakeLists.txt b/libcxxabi/CMakeLists.txt
index b23936e..5eb0109 100644
--- a/libcxxabi/CMakeLists.txt
+++ b/libcxxabi/CMakeLists.txt
@@ -249,6 +249,10 @@
 add_target_flags_if(LIBCXXABI_SYSROOT
           "--sysroot=${LIBCXXABI_SYSROOT}")
 
+if (LIBCXXABI_TARGET_TRIPLE)
+  set(TARGET_TRIPLE "${LIBCXXABI_TARGET_TRIPLE}")
+endif()
+
 # Configure compiler. Must happen after setting the target flags.
 include(config-ix)