[libc++abi] lack of the detection of libunwind library files.

[ye-luo]

Triggered by #77687
libc++abi doesn't seem have strong enough dependency check/handling of libunwind.

cmake line

cmake -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DCMAKE_BUILD_TYPE=Release -DGCC_INSTALL_PREFIX=$GCC_ROOT -DCMAKE_INSTALL_PREFIX=$INSTALL_FOLDER -DLLVM_ENABLE_BACKTRACES=ON -DLLVM_ENABLE_WERROR=OFF -DBUILD_SHARED_LIBS=OFF -DLLVM_ENABLE_RTTI=ON -DLLVM_ENABLE_ASSERTIONS=ON -DLLVM_ENABLE_PROJECTS="clang;compiler-rt;lld;openmp;flang;mlir" -DLLVM_ENABLE_RUNTIMES="libcxxabi;libcxx" -DOPENMP_ENABLE_LIBOMPTARGET=ON -DLIBOMPTARGET_ENABLE_DEBUG=ON ../llvm-project-nightly/llvm

My local build stops at

/usr/bin/ld: cannot find -lunwind_shared: No such file or directory
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
make[5]: *** [libcxxabi/src/CMakeFiles/cxxabi_shared.dir/build.make:117: /gpfs/jlse-fs0/users/yeluo/opt/llvm-clang/build_mirror_offload_nightly/lib/x86_64-unknown-linux-gnu/libc++abi.so.1.0] Error 1

this kind of issue should be detected in the cmake stage of libcxxabi not at compile time.