[cmake] [libcxx] Fix find_path() problems when cross compiling.

When CMAKE_SYSROOT or CMAKE_FIND_ROOT_PATH is set, cmake
recommends setting CMAKE_FIND_ROOT_PATH_MODE_INCLUDE=ONLY
globally which means find_path() always prepends CMAKE_SYSROOT or
CMAKE_FIND_ROOT_PATH to all paths used in the search.

However, this find_path() invocation is looking for a path in the
libcxxabi project on the host system, not the target system,
which can be done by passing NO_CMAKE_FIND_ROOT_PATH.

Differential Revision: https://reviews.llvm.org/D41622

llvm-svn: 323143

libcxx/CMakeLists.txt

@@ -123,6 +123,7 @@ if (LIBCXX_CXX_ABI STREQUAL "default")
           ${LLVM_MAIN_SRC_DIR}/runtimes/libcxxabi/include
           ${LLVM_MAIN_SRC_DIR}/../libcxxabi/include
     NO_DEFAULT_PATH
+    NO_CMAKE_FIND_ROOT_PATH
   )
   if (LIBCXX_TARGETING_MSVC)
     # FIXME: Figure out how to configure the ABI library on Windows.