Bug: http://b/31320715
Use LLVM's RenderScript triples: upstream LLVM now has 32-bit and
64-bit RenderScript triples. Update RS_TRIPLE build variables to use
the new triples.
Switch prebuilt clang used for building RenderScript runtime library to
the latest prebuilts.
Test: Run RenderScript tests (slang, bcc host tests, RsTest, CTS)
Change-Id: I308f6a2183350cfc380228a1eed07ec118d93a46