Change STL for Windows to libc++.

- Link libgcc_eh, which needs pthread, which in turn depends on
kernel32.  Wrap the libraries with --start-group and --end-group and
remove duplicates.
- 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).

Test: Build and test Windows binaries under Wine.

Change-Id: I594fa7673282ff5c1b7b80d49f48b7618960a148