Find libc++ in the per-target runtime directory

[eugenis]

Summary

Fixes build after "[CMake] Enable LLVM_ENABLE_PER_TARGET_RUNTIME_DIR by
default on Linux.

[eugenis]

Breaking revision: https://reviews.llvm.org/D107799

It would be nice if something like -print-runtime-dir, but we don't even build clang in this directory.

[vitalybuka]

we probably can install libcxx into dir we want

[MaskRay]

Thanks for investigating the issue:)

[eugenis]

I've also added -DLLVM_ENABLE_PER_TARGET_RUNTIME_DIR=OFF in a follow-up commit because there are more issues.

zorg/buildbot/builders/sanitizers/buildbot_functions.sh

function build_stage2 {
   (cd ${libcxx_build_dir} && \
     cmake \
       ${cmake_stage2_common_options} \
       -DLLVM_ENABLE_PROJECTS='libcxx;libcxxabi' \
       -DCMAKE_BUILD_TYPE=${build_type} \
       -DLLVM_USE_SANITIZER=${llvm_use_sanitizer} \
       $LLVM && \
     ninja cxx cxxabi) || echo $step_result
-  local sanitizer_ldflags="-lc++abi -Wl,--rpath=${ROOT}/${libcxx_build_dir}/lib -L${ROOT}/${libcxx_build_dir}/lib"
+
+  libcxx_runtime_path=$(dirname $(find ${ROOT}/${libcxx_build_dir} -name libc++.so))
+  local sanitizer_ldflags="-lc++abi -Wl,--rpath=${libcxx_runtime_path} -L${libcxx_runtime_path}"
   local sanitizer_cflags="-nostdinc++ -isystem ${ROOT}/${libcxx_build_dir}/include -isystem ${ROOT}/${libcxx_build_dir}/include/c++/v1"
 
   echo @@@BUILD_STEP stage2/$sanitizer_name build@@@
 
   # See http://llvm.org/bugs/show_bug.cgi?id=19071, http://www.cmake.org/Bug/view.php?id=15264
   local cmake_bug_workaround_cflags="$sanitizer_ldflags $fsanitize_flag -w"
   sanitizer_cflags="$sanitizer_cflags $cmake_bug_workaround_cflags"