Make native-xtools build correctly after clang/llvm 3.9.0 import

During the clang/llvm 3.9.0 import, the build structure for it was
completely revamped. This broke the native-xtools target.

It first attempts to build libllvmminimal, then the llvm-tblgen and
clang-tblgen executables, but these fail to link because they are linked
to the 'full' libllvm by default, as they normally are during the
'world' stage.

To make these link against libllvmminimal instead, define TOOLS_PREFIX,
similarly as during the bootstrap-tools phase. The value itself is
empty, as we don't really want to use a prefix.

Reviewed by: imp
PR: 215684
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D9026