[ppc64] Enable sibling call optimization on ppc64 ELFv1/ELFv2 abi

This patch enable sibling call optimization on ppc64 ELFv1/ELFv2 abi, and
add a couple of test cases. This patch also passed llvm/clang bootstrap
test, and spec2006 build/run/result validation.

Original issue: https://llvm.org/bugs/show_bug.cgi?id=25617

Great thanks to Tom's (tjablin) help, he contributed a lot to this patch.
Thanks Hal and Kit's invaluable opinions!

Reviewers: hfinkel kbarton

http://reviews.llvm.org/D16315

llvm-svn: 265506