[libcxxabi] [test] Fix the mingw test config

Don't link libc++abi separately in addition to the main -lc++; in
mingw build configs, libc++abi is always bundled into libc++
(via LIBCXX_ENABLE_STATIC_ABI_LIBRARY).

In the case of a shared linked libc++, linking a separate static
libc++abi leads to linker errors.

Differential Revision: https://reviews.llvm.org/D147638