Fix the linked targets for rustc

Right now multiple targets/hosts is broken because the libdir passed for all of
the LLVM libraries is for the wrong architecture. By using the right arch
(target, not host), everything is linked and assembled just fine.
auto merge of #10850 : alexcrichton/rust/fix-target, r=pcwalton

Right now multiple targets/hosts is broken because the libdir passed for all of
the LLVM libraries is for the wrong architecture. By using the right arch
(target, not host), everything is linked and assembled just fine.