Try to fix the libcxx build with mingw64

[rnk]

Summary

mingw64 has lots of default libs that libc++ and its test programs
should link against.

With this patch, cmake now runs successfully with GCC on Windows.

[EricWF]

LGTM. CMake's behavior here is unfortunate but this seems like a reasonable workaround.

libcxx/cmake/config-ix.cmake

if (LIBCXX_SUPPORTS_NODEFAULTLIBS_FLAG)
   if (LIBCXX_HAS_C_LIB)
     list(APPEND CMAKE_REQUIRED_LIBRARIES c)
   endif ()
   if (LIBCXX_USE_COMPILER_RT)
     list(APPEND CMAKE_REQUIRED_LIBRARIES -rtlib=compiler-rt)
   elseif (LIBCXX_HAS_GCC_S_LIB)
     list(APPEND CMAKE_REQUIRED_LIBRARIES gcc_s)
   endif ()
+  if (MINGW)
+    # Mingw64 requires quite a few "C" runtime libraries in order for basic
+    # programs to link successfully with -nodefaultlibs.
+    list(APPEND CMAKE_REQUIRED_LIBRARIES mingw32 gcc gcc_eh mingwex msvcrt gcc)
+  endif()
   if (CMAKE_C_FLAGS MATCHES -fsanitize OR CMAKE_CXX_FLAGS MATCHES -fsanitize)
     set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -fno-sanitize=all")
   endif ()
   if (CMAKE_C_FLAGS MATCHES -fsanitize-coverage OR CMAKE_CXX_FLAGS MATCHES -fsanitize-coverage)
     set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -fno-sanitize-coverage=edge,trace-cmp,indirect-calls,8bit-counters")
   endif ()
 endif ()