Add MRegisterInfo subclass for the SparcV9 target (containing only stub

functions for now). This automatically turns on the printing of machine
registers using their own real names, instead of goofy things like %mreg(42),
and allows us to migrate code incrementally to the new interface as we see fit.

The register file description it uses is hand-written, so that the register
numbers will match the ones that the SparcV9 target already uses.
Perhaps someday we'll tablegen it.

llvm-svn: 13145