[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

libcxxabi/test/configs/llvm-libc++abi-mingw.cfg.in

@@ -8,7 +8,7 @@ config.substitutions.append(('%{compile_flags}',
     '-nostdinc++ -I %{include} -I %{cxx-include} -I %{cxx-target-include} %{maybe-include-libunwind} -I %{libcxx}/test/support -I %{libcxx}/src -D_LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS'
 ))
 config.substitutions.append(('%{link_flags}',
-    '-nostdlib++ -L %{lib} -lc++ -lc++abi'
+    '-nostdlib++ -L %{lib} -lc++'
 ))
 config.substitutions.append(('%{exec}',
     '%{executor} --execdir %T --env PATH=%{lib} -- '