For X86-64 linux and PPC64 linux align int128 to 16 bytes.

For other platforms we should find out what they need and likely
make the same change, however, a smaller additional change is easier
for platforms we know have it specified	in the ABI. As part of this
rewrite some of the handling in the backends for data layout and update
a bunch of testcases.

Based on a patch by Simonas Kazlauskas!

llvm-svn: 294702