Fix the failures in the PPC JIT by marking relocation entries for

external symbols (e.g. 'fmod') as needing a stub.  This regression
was introduced by Evan's jit patch here:
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20071231/056749.html

With this fixed, the two ExecutionEngine failures are passing on ppc,
and the ppc jit works on freebench and olden.

This should be pulled into the 2.2 release branch.

llvm-svn: 46222