[libcxx] Link MinGW libs for shared build

[mati865]

Summary

Another small step for libc++ with MinGW-w64.

[martell]

Not quite sure how this achieves anything different from what is already in cmake/config-ix.cmake ?
D33638 addresses the remaining changes needed to build for windows shared and static in one build.

[mati865]

Sorry @martell I totally forgot about it.
I'll test D33638 later today.

lib/CMakeLists.txt

if (LIBCXX_TARGETING_MSVC)
 
   add_library_flags(ucrt${LIB_SUFFIX}) # Universal C runtime
   add_library_flags(vcruntime${LIB_SUFFIX}) # C++ runtime
   add_library_flags(msvcrt${LIB_SUFFIX}) # C runtime startup files
   add_library_flags(msvcprt${LIB_SUFFIX}) # C++ standard library. Required for exception_ptr internals.
   # Required for standards-complaint wide character formatting functions
   # (e.g. `printfw`/`scanfw`)
   add_library_flags(iso_stdio_wide_specifiers)
+elseif(MINGW)
+  if (LIBCXX_USE_COMPILER_RT)
+    set(MINGW_RUNTIME ${LIBCXX_BUILTINS_LIBRARY})
+  else ()
+    set(MINGW_RUNTIME gcc_s gcc)
+  endif()
+  add_library_flags(mingw32)
+  add_library_flags(${MINGW_RUNTIME})
+  add_library_flags(moldname)
+  add_library_flags(mingwex)
+  add_library_flags(msvcrt)
+  add_library_flags(advapi32)
+  add_library_flags(shell32)
+  add_library_flags(user32)
+  add_library_flags(kernel32)
+  add_library_flags(mingw32)
+  add_library_flags(${MINGW_RUNTIME})
+  add_library_flags(moldname)
+  add_library_flags(mingwex)
+  add_library_flags(msvcrt)
 endif()
 
 if (LIBCXX_OSX_REEXPORT_SYSTEM_ABI_LIBRARY)
   if (NOT DEFINED LIBCXX_LIBCPPABI_VERSION)
     set(LIBCXX_LIBCPPABI_VERSION "2") # Default value
     execute_process(
       COMMAND xcrun --show-sdk-version
       OUTPUT_VARIABLE sdk_ver