Remove implicit fallthrough from AArch64 to Mips in lib selection

Nothing useful to AArch64 will (should!) be found in any Mips-specific
directories.

Patch by Luke Zarko.

llvm-svn: 183956
diff --git a/clang/lib/Driver/ToolChains.cpp b/clang/lib/Driver/ToolChains.cpp
index 1a7eb1b..71b010e 100644
--- a/clang/lib/Driver/ToolChains.cpp
+++ b/clang/lib/Driver/ToolChains.cpp
@@ -2153,6 +2153,7 @@
   case llvm::Triple::aarch64:
     if (llvm::sys::fs::exists(SysRoot + "/lib/aarch64-linux-gnu"))
       return "aarch64-linux-gnu";
+    return TargetTriple.str();
   case llvm::Triple::mips:
     if (llvm::sys::fs::exists(SysRoot + "/lib/mips-linux-gnu"))
       return "mips-linux-gnu";