[libc++] Explicitly specify LIBCXX_ENABLE_SHARED to try and fix CI

Configuring libc++abi with LIBCXX_ENABLE_STATIC=OFF is broken since
https://reviews.llvm.org/D71894, so this patch fixes the issue for
Apple platforms to unblock our CI.

libcxx/cmake/caches/Apple.cmake

@@ -8,6 +8,7 @@ set(LIBCXX_ABI_VERSION "1" CACHE STRING "")
 
 set(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY OFF CACHE BOOL "")
 set(LIBCXX_ENABLE_STATIC OFF CACHE BOOL "")
+set(LIBCXX_ENABLE_SHARED ON CACHE BOOL "")
 
 set(LIBCXX_ENABLE_NEW_DELETE_DEFINITIONS OFF CACHE BOOL "")
 set(LIBCXXABI_ENABLE_NEW_DELETE_DEFINITIONS ON CACHE BOOL "")