exclude src/opts/SkXbyak.cpp from g3 build

Change-Id: I5027275446ab9cc7fc7b12b2236a9d85ac50bef1
Reviewed-on: https://skia-review.googlesource.com/6695
Reviewed-by: Ben Wagner <benjaminwagner@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

public.bzl

@@ -139,6 +139,9 @@ BASE_SRCS_ALL = struct(
 
         # Defines main.
         "src/sksl/SkSLMain.cpp",
+
+        # Experiment.  Google3 doesn't have the library it needs.
+        "src/opts/SkXbyak.cpp",
     ],
 )