Add basic 64-bit SPARC support

Add support for the most common SPARC relocations.
Make DT_PLTGOT point to the PLT on SPARC.
Mark the PLT as executable on SPARC.

This adds a basic test that creates a SPARV9 executable
that invokes the exit system call on OpenBSD.

Patch by Mark Kettenis.

Differential Revision: https://reviews.llvm.org/D34618

llvm-svn: 306565