Compile varargs functions for SPARCv9.

With a little help from the frontend, it looks like the standard va_*
intrinsics can do the job.

Also clean up an old bitcast hack in LowerVAARG that dealt with
unaligned double loads. Load SDNodes can specify an alignment now.

Still missing: Calling varargs functions with float arguments.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179961 91177308-0d34-0410-b5e6-96231b3b80d8