Centralized symbol lookup in IRExecutionUnit, and fixed the code model.

I'm preparing to remove symbol lookup from IRForTarget, where it constitutes a
dreadful hack working around no-longer-existing JIT bugs.  Thanks to our 
contributors, IRForTarget has a lot of smarts that IRExecutionUnit doesn't have,
so I've cleaned them up a bit and moved them over to IRExecutionUnit.

Also for historical reasons, IRExecutionUnit used the "Small" code model on non-
ELF platforms (namely, OS X).  That's no longer necessary, and we can use the
same code model as everyone else on OS X.  I've fixed that.

llvm-svn: 260734