Allow overriding the LLVM prebuilts path.

The automated build will need to do a two stage build, and we want to
do that without altering the source directory. Building with
`make LLVM_PREBUILTS_BASE=/path/to/prebuilts` will build using an
arbitrary toolchain instead of the installed one.

Bug: http://b/17441393
Change-Id: I4ff5c5224fbb8ebeb3a752907d6c4cd0a925aa1e