Migrate llc and the JIT to using the TargetRegistry for lookups.

 - They still use the TargetMachineRegistry to populate the contents of the
   -march option (via the listener interface). We can't just populate it in the
   option parser because we can't expect the TargetRegistry to be populated yet
   (we no longer rely on static constructors).

 - There are a couple ways to finish killing off TargetMachineRegistry, but I
   haven't figured out the cleanest one yet...

llvm-svn: 75773