This checkin represents some cleanup of the backend, implementing the following things:

1. The TargetMachine structure is free to decide the process a particular target uses to generate code.
2. All of the gooee details of the sparc backend are now localized in the lib/CodeGen/TargetMAchine/Sparc directory.  The Sparc.h file that is globally visible is just a stub.
3. The Sparc.h file that esxists now will dissapear entirely someday when we have multiple backends chosen by a factory of some sort.

llvm-svn: 559