[Nios2] Target registration

Reviewers: craig.topper, hfinkel, joerg, lattner, zvi

Reviewed By: craig.topper

Subscribers: oren_ben_simhon, igorb, belickim, tvvikram, mgorny, llvm-commits, pavel.v.chupin, DavidKreitzer

Differential Revision: https://reviews.llvm.org/D32669
Patch by AndreiGrischenko <andrei.l.grischenko@intel.com>

llvm-svn: 304144