Revert "Disabled libunwind, #96 was not enough"

Looks like llvm/llvm-project#77689 fixed the issue.

This reverts commit 23cd70e.

Author: vitalybuka

zorg/buildbot/builders/sanitizers/buildbot_functions.sh

@@ -261,7 +261,6 @@ function build_stage2 {
     cmake \
       ${cmake_stage2_common_options} \
       -DLLVM_ENABLE_RUNTIMES='libcxx;libcxxabi;libunwind' \
-      -DLIBCXXABI_USE_LLVM_UNWINDER=OFF \
       -DLLVM_USE_SANITIZER=${llvm_use_sanitizer} \
       -DCMAKE_C_FLAGS="${fsanitize_flag} ${cmake_libcxx_cflags} ${fno_sanitize_flag}" \
       -DCMAKE_CXX_FLAGS="${fsanitize_flag} ${cmake_libcxx_cflags} ${fno_sanitize_flag}" \