[Nios2] Subtarget, basic infrastructure for frame, instructions and registers

This is the second minimal patch keeping Nios2 target buildable.
I'm adding subtarget here and other stuff for frame lowering, instruction,
register information methods. I do not add any test cases, as still there
are missing parts like DAG selector and assembly printing. I plan to include
them into the next patch.

Patch by Andrei Grischenko <andrei.l.grischenko@intel.com>

Differential Revision: https://reviews.llvm.org/D37256

llvm-svn: 313626