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
diff --git a/core/clang/TARGET_mips64.mk b/core/clang/TARGET_mips64.mk
index 785752a..a3684cc 100644
--- a/core/clang/TARGET_mips64.mk
+++ b/core/clang/TARGET_mips64.mk
@@ -1,4 +1,4 @@
-RS_TRIPLE := aarch64-linux-android
+RS_TRIPLE := renderscript64-linux-android
 RS_TRIPLE_CFLAGS :=
 RS_COMPAT_TRIPLE := mips64el-linux-android