be more explicit about not expecting 32-bit x86 jumper backend on windows

Looks like Clang/Win is defining __i386__, but we're not linking in
stage functions (they don't exist yet for Windows).

Change-Id: I78fdd3e1d89020bc6c64bc1cd5dfb3fbca720b2e
Reviewed-on: https://skia-review.googlesource.com/21103
Commit-Queue: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
1 file changed