Add stack alignment information for Sparc.

This matches the data in clang which was added by Jakob Stoklund Olesen in
r179596.

Thanks for erikjv on irc for pointing me to the relevant documents:
http://sparc.com/standards/64.psabi.1.35.ps.Z
page 25: Every stack frame must be 16-byte aligned.

http://sparc.com/standards/psABI3rd.pdf
page 3-10: Although the architecture requires only word alignment, software convention and the operating system require every stack frame to be doubleword aligned.

I tried to add a test, but it looks like sparc doesn't implement dynamic stack
realignment. This will be tested in clang shortly.