Fix tracing code support to link in the tracing routines if neccesary.

Fix tracing code support to link in the tracing routines if neccesary.
This is required because trace code can insert calls to functions that
are (now) in an LLVM library, not a native library.

This change makes it easier to use the tracing code because the native
link options don't have to change, and it fits in a lot better with the
LLVM model.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@2675 91177308-0d34-0410-b5e6-96231b3b80d8