[Sparc] Use the leon arch for Leon3's when using an external assembler

Summary: This allows the use of the casa instruction available in most Leon3's.

Reviewers: jyknight

Reviewed By: jyknight

Subscribers: joerg, fedor.sergeev, jrtc27, cfe-commits

Differential Revision: https://reviews.llvm.org/D47138

llvm-svn: 333157