Enable libcxx_static for Windows

- Use Win32 threads.  MinGW pthreads throws an error when building
libcxx since it's pthread_mutex_initializer is not constant (needs a
cast).
- Disable visibility annotations for libcxx and libcxxabi since we are
only going to support these as static libraries.
- Build sources for Windows platform support.
- Use SjLj exceptions for 32-bit.  libgcc_eh implements SjLj exception
model for 32-bit.
- Undefine WIN32_LEAN_AND_MEAN macro.  libcxx sources define them where
necessary.

Test: Build and test Windows binaries under Wine.

Change-Id: I76a440ff5d71eb5a3b7d57dd97e742a458ffbe16

Android.bp

@@ -70,6 +70,24 @@ cc_library_static {
         linux_bionic: {
             enabled: true,
         },
+        windows: {
+            enabled: true,
+            cflags: [
+                "-D_LIBCPP_HAS_THREAD_API_WIN32",
+                "-D_LIBCXXABI_BUILDING_LIBRARY",
+                "-D_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS",
+                "-D_LIBCXXABI_DISABLE_VISIBILITY_ANNOTATIONS",
+                "-UWIN32_LEAN_AND_MEAN",
+            ],
+            srcs: [
+                "src/support/win32/*.cpp",
+            ]
+        },
+        windows_x86: {
+            cflags: [
+                "-fsjlj-exceptions",
+            ],
+        },
     },
 }