Move the registered target printing in version strings completely out of

the Support library. Now its part of the TargetRegistry, and the three
commands that care about this explicitly register this extra bit of
version information.

The set of commands which care was computed by intersecting those which
use the Support library's version string printing and those that
initialize all the registered targets in a way that produces
a meaningful list. The only odd ball out is that 'clang -cc1as -version'
no longer prints the registered targets. I don't think anyone is really
interested in that (especially as the fact that llvm-mc does so is under
a FIXME), but if someone really does want this back I'll happily apply
the same patch there.

llvm-svn: 135757