[libcxx][ci] Switch to CMAKE_CXX_COMPILER_TARGET for Arm bots

As suggested by the cmake warning:
CMake Warning at <...>/llvm-project/libcxx-ci/libcxx/CMakeLists.txt:289 (message):
  LIBCXX_TARGET_TRIPLE is deprecated, please use CMAKE_CXX_COMPILER_TARGET instead

Depends on D119948

Differential Revision: https://reviews.llvm.org/D120038