update our build system to support the experimental L-Release NDK

This includes adding 64-bit support for arm, x86, and mips.

R=borenet@google.com

Review URL: https://codereview.chromium.org/377993002
diff --git a/gyp/gpu.gyp b/gyp/gpu.gyp
index 6fbd05a..c1da4f1 100644
--- a/gyp/gpu.gyp
+++ b/gyp/gpu.gyp
@@ -215,6 +215,9 @@
             '../src/gpu/gl/GrGLDefaultInterface_none.cpp',
             '../src/gpu/gl/GrGLCreateNativeInterface_none.cpp',
           ],
+          'defines': [
+            'GR_GL_USE_NEW_SHADER_SOURCE_SIGNATURE=1',
+          ],
           'link_settings': {
             'libraries': [
               '-lGLESv2',