Use "${config.ClangBin}/llvm-" for TOOLS_PREFIX.

We're removing GNU binutils in Android T, so lets redirect any last
remaining users to the LLVM equivalents sooner rather than later.

(This won't "just work" for ld versus lld, but it's likely that anyone
using TOOLS_PREFIX will need a few other tweaks anyway, and including
the "llvm-" prefix seems like it will cover most other cases.)

Bug: http://b/185257607
Test: treehugger
Change-Id: Ibb5efbbef801419e7d8258d6d8b67572df66e7d0