CompilerRT: Normalize COMPILER_RT_DEFAULT_TARGET_TRIPLE (#88835)

If LLVM is configured with -DLLVM_DEFAULT_TARGET_TRIPLE, or compiler_rt
is configured with -DCOMPILER_RT_DEFAULT_TARGET_TRIPLE, while the
argument is not normalized, such as Debian-style vendor-less triple,
clang will try to find libclang_rt in lib/<normalized_triple>, while
libclang_rt is placed into lib/<triple_arg>.

Let's also place libclang_rt into lib/<normalized_triple>.