Exclude SkJumper_stages_lowp.cpp from G3 build.

It's not meant to be built as part of Skia,
only by build_stages.py into SkJumper_generated.S.

Change-Id: Id028c42355f99415fe2bc0710c2292ea949f6eec
Reviewed-on: https://skia-review.googlesource.com/18593
Reviewed-by: Mike Klein <mtklein@chromium.org>
Reviewed-by: Ben Wagner <benjaminwagner@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

public.bzl

@@ -117,6 +117,9 @@ BASE_SRCS_ALL = struct(
 
         # Defines main.
         "src/sksl/SkSLMain.cpp",
+
+        # Only pre-compiled into SkJumper_generated.S.
+        "src/jumper/SkJumper_stages_lowp.cpp",
     ],
 )