Use new variable LLVM_PREBUILTS_VERSION to simplify path construction.

This removes the hard-coded '3.5' in the various paths and makes switching to
a new toolchain easier from the command line (m LLVM_PREBUILTS_VERSION=3.6).

Change-Id: I46b10eb2fc177a03528de9c7b433f8647f632081
1 file changed