Statically link libc++abi into libc++ (#483)

This will prevent C++ links from breaking when
llvm/llvm-project#101259 lands, as that removes
-lc++abi and -lunwind from the linker command-line.

Author: ostannard

CMakeLists.txt

@@ -1201,6 +1201,7 @@ function(
         -DLIBCXXABI_TEST_PARAMS=executor=${test_executor}
         -DLIBCXX_ABI_UNSTABLE=ON
         -DLIBCXX_CXX_ABI=libcxxabi
+        -DLIBCXX_STATICALLY_LINK_ABI_IN_STATIC_LIBRARY=ON
         -DLIBCXX_ENABLE_FILESYSTEM=OFF
         -DLIBCXX_ENABLE_SHARED=OFF
         -DLIBCXX_ENABLE_STATIC=ON