[Sparc] Allow LEON cpu models to be selected with -mcpu

Summary: This change exposes the recently added LEON CPUs (D19359) in the LLVM Sparc backend to Clang, allowing the cpu's to be selected using the -mcpu flag.

Reviewers: jyknight, lero_chris

Subscribers: jyknight, cfe-commits

Differential Revision: http://reviews.llvm.org/D21683

llvm-svn: 274487