Update SwiftShader Kokoro build (google#516)

Upstream has switched from -fvisibliity=protected to -Bsymbolic. The
former was triggering a linker bug which required the -fuse-ld=gold
workaround (which can now be removed). The latter triggers a Clang
warning about the linker argument being unused, so we now need to
disable -Werror.

Also updates the CMake flags used to disable various backends.

kokoro/scripts/linux/build-clvk.sh

@@ -115,10 +115,9 @@ SWIFTSHADER_BUILD="$SWIFTSHADER_SRC/kokoro-build"
 git clone https://swiftshader.googlesource.com/SwiftShader.git "$SWIFTSHADER_SRC"
 cd "$SWIFTSHADER_SRC" && git submodule update --init
 mkdir "$SWIFTSHADER_BUILD" && cd "$SWIFTSHADER_BUILD"
-# Use gold to workaround an issue with GNU ld v2.26
 cmake -DPYTHON_EXECUTABLE:FILEPATH=/usr/bin/python3 -GNinja -DCMAKE_BUILD_TYPE=$BUILD_TYPE $ADDITIONAL_CMAKE_FLAGS $CMAKE_C_CXX_COMPILER \
-  -DCMAKE_SHARED_LINKER_FLAGS="-fuse-ld=gold" \
-  -DBUILD_VULKAN=ON -DBUILD_EGL=OFF -DBUILD_GLESv2=OFF -DBUILD_GLES_CM=OFF -DBUILD_SAMPLES=OFF -DBUILD_TESTS=OFF "$SWIFTSHADER_SRC"
+      -DSWIFTSHADER_WARNINGS_AS_ERRORS=OFF  -DSWIFTSHADER_BUILD_VULKAN=ON -DSWIFTSHADER_BUILD_EGL=OFF -DSWIFTSHADER_BUILD_GLESv2=OFF \
+      -DSWIFTSHADER_BUILD_GLES_CM=OFF -DSWIFTSHADER_BUILD_SAMPLES=OFF -DSWIFTSHADER_BUILD_TESTS=OFF "$SWIFTSHADER_SRC"
 ninja
 echo $(date): SwiftShader build completed.