[wasm][build] Disable LLVM unwinder in libcxxabi (#77018)

This change disables the use of llvm libunwind in libcxxabi, which is enabled by default in the rebranched scheme. (https://reviews.llvm.org/D150897) But the llvm libunwind is not supported in the WebAssembly target, so we need to disable it explicitly.

This is a preparation for the upcoming `rebranch` merge.

Author: kateinoigakukun

utils/swift_build_support/swift_build_support/products/wasisysroot.py

@@ -209,6 +209,7 @@ def _build(self, host_target, enable_wasi_threads=False,
         self.cmake_options.define('LIBCXX_ABI_VERSION', '2')
         self.cmake_options.define('LIBCXXABI_ENABLE_EXCEPTIONS:BOOL', 'FALSE')
         self.cmake_options.define('LIBCXXABI_ENABLE_SHARED:BOOL', 'FALSE')
+        self.cmake_options.define('LIBCXXABI_USE_LLVM_UNWINDER:BOOL', 'FALSE')
         self.cmake_options.define('LIBCXXABI_SILENT_TERMINATE:BOOL', 'TRUE')
         self.cmake_options.define('LIBCXXABI_ENABLE_THREADS:BOOL', cmake_has_threads)
         self.cmake_options.define('LIBCXXABI_HAS_PTHREAD_API:BOOL', cmake_has_threads)