SkJumper: Windows

    - Compile stages with -DWIN to pick up MS-specific start_pipeline().
    - Add SkJumper_generated_win.S with MS-specific assembly.
    - Add a minimal asm tool to our GN Windows toolchain.

The SkRasterPipeline_f16 benchmark run ~4x faster on my desktop.

Change-Id: Ia45afb4ecb6a055e2c0e43f0f54f59e081c23b7f
Reviewed-on: https://skia-review.googlesource.com/8778
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
7 files changed