[Sparc] Use target name instead of namespace as prefix for MCRegisterClasses array

Summary:
For Sparc the namespace (SP) is different from the target name (Sparc),
which causes the name of the array in this declaration to differ from
the name used in the definition.

Patch by Daniel Cederman.

Reviewers: jyknight

Subscribers: llvm-commits, jyknight

Differential Revision: https://reviews.llvm.org/D23650

llvm-svn: 287528