Change STL for Windows to libc++.

- Stop including libstdc++ headers.
- '-pthread' and 'static-libgcc' are unused when we pass -nodefaultlibs.
We didn't pass -nodefaultlibs for libstdc++.
- Use SjLj exceptions for 32-bit.  libgcc_eh implements SjLj exception
model for 32-bit.
- Disable visibility annotations for libcxx and libcxxabi since we are
only going to support these as static libraries.
- Use Win32 threads.  MinGW pthreads throws an error when building
libcxx since it's pthread_mutex_initializer is not constant (needs a
cast).
- Link libgcc_eh, which needs pthread, which in turn depends on
kernel32.  Wrap the libraries with --start-group and --end-group and
remove duplicates.

Test: Build and test Windows binaries under Wine.

Change-Id: I8be51b004585e11ef51b7d5012f2a51330d1260f