This patch adds alignment information for long double to the 64-bit PowerPC

This patch adds alignment information for long double to the 64-bit PowerPC
ELF subtarget.

The existing logic is used as a fallback to avoid any changes to the Darwin
ABI.  PPC64 ELF now has two possible data layout strings: one for FreeBSD,
which requires 8-byte alignment, and a default string that requires
16-byte alignment.

I've added a test for PPC64 Linux to verify the 16-byte alignment.  If
somebody wants to add a separate test for FreeBSD, that would be great.

Note that there is a companion patch to update the alignment information
in Clang, which I am committing now as well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@166928 91177308-0d34-0410-b5e6-96231b3b80d8