RenderScript-specific build updates for LLVM rebase

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