add 32-bit Windows SkJumper backend

The most interesting part of this is getting the call to start_pipeline
to work.  From there it should be just like the other x86 backend.

The 32-bit calling conventions are the same across Linux/Mac and
Windows, so that's nice.  The tricky bit is that Linux and Mac
align the stack to 16 bytes, while Windows only to 4.  I think
this force_align_arg_pointer attribute on start_pipeline does the trick.

This needs a guard for layout tests.

CQ_INCLUDE_TRYBOTS=skia.primary:Test-Win2k8-MSVC-GCE-CPU-AVX2-x86-Debug;master.tryserver.blink:win10_blink_rel,win7_blink_rel;master.tryserver.chromium.win:win_chromium_rel_ng

Change-Id: Ia74d22e5a4ce5483c9817b8a8f89dd21885bbd14
Reviewed-on: https://skia-review.googlesource.com/20968
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Reed <reed@google.com>
4 files changed