[libcxx] [docs] Update the MinGW build example (#68790)

The previous example wasn't a configuration that we actually test, but
was a simplistic configuration with libstdc++ as ABI library.

The previous example configuration has bitrotted and broken in a couple
different ways since it was added:

- In b0fd949, libcxx added uses of weak
  symbols that can be overridden by the user. GNU ld fails to export such
  weak symbols on MinGW, resulting in errors like:
    C:/msys64/mingw64/bin/ld: cannot export _ZNSt3__126__libcpp_assertion_handlerEPKciS1_S1_: symbol wrong type (2 vs 3) 
  By switching to using LLD, -DLLVM_ENABLE_LLD=ON, alternatively
  -DCMAKE_SHARED_LINKER_FLAGS=-fuse-ld=lld, linking still succeeds.

- In a80e65e, building with
  -DLIBCXX_CXX_ABI=libstdc++ in this configuration started failing. Switch
  to showing how to build with libcxxabi instead, which is the tested
  configuration and also is the typical configuration in how it most
  usually would be used on Windows.

These updates allow removing the mentions of the old caveats with the
build configuration.

Author: mstorsjo

libcxx/docs/BuildingLibcxx.rst

@@ -157,19 +157,13 @@ e.g. the ``mingw-w64-x86_64-clang`` package), together with CMake and ninja.
   > cmake -G Ninja -S runtimes -B build                                               \
           -DCMAKE_C_COMPILER=clang                                                    \
           -DCMAKE_CXX_COMPILER=clang++                                                \
-          -DLLVM_ENABLE_RUNTIMES=libcxx                                               \
-          -DLIBCXX_CXX_ABI=libstdc++
+          -DLLVM_ENABLE_LLD=ON                                                        \
+          -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi"                                   \
+          -DLIBCXXABI_ENABLE_SHARED=OFF                                               \
+          -DLIBCXX_ENABLE_STATIC_ABI_LIBRARY=ON
   > ninja -C build cxx
-  > cp /mingw64/bin/{libstdc++-6,libgcc_s_seh-1,libwinpthread-1}.dll lib
   > ninja -C build check-cxx
 
-As this build configuration ends up depending on a couple other DLLs that
-aren't available in path while running tests, copy them into the same
-directory as the tested libc++ DLL.
-
-(Building a libc++ that depends on libstdc++ isn't necessarily a config one
-would want to deploy, but it simplifies the config for testing purposes.)
-
 .. _`libc++abi`: http://libcxxabi.llvm.org/